White Dwarf Pulses Like A Pulsar

Daily Science Journal (Jan. 3, 2008) — New observations from Suzaku, a joint Japanese Aerospace Exploration Agency (JAXA) and NASA X-ray observatory, have challenged scientists’ conventional understanding of white dwarfs. Observers had believed white dwarfs were inert stellar corpses that slowly cool and fade away, but the new data tell a completely different story.

The white dwarf in the AE Aquarii system is the first star of its type known to give off pulsar-like pulsations that are powered by its rotation and particle acceleration. (Credit: Casey Reed)

At least one white dwarf, known as AE Aquarii, emits pulses of high-energy (hard) X-rays as it whirls around on its axis. "We’re seeing behavior like the pulsar in the Crab Nebula, but we’re seeing it in a white dwarf," says Koji Mukai of NASA Goddard Space Flight Center in Greenbelt, Md. The Crab Nebula is the shattered remnant of a massive star that ended its life in a supernova explosion. "This is the first time such pulsar-like behavior has ever been observed in a white dwarf." Mukai is co-author of a paper presented at a Suzaku science conference in San Diego, Calif., in December.

White dwarfs and pulsars represent distinct classes of compact objects that are born in the wake of stellar death. A white dwarf forms when a star similar in mass to our sun runs out of nuclear fuel. As the outer layers puff off into space, the core gravitationally contracts into a sphere about the size of Earth, but with roughly the mass of our sun. The white dwarf starts off scorching hot from the star’s residual heat. But with nothing to sustain nuclear reactions, it slowly cools over billions of years, eventually fading to near invisibility as a black dwarf.

A pulsar is a type of neutron star, a collapsed core of an extremely massive star that exploded in a supernova. Whereas white dwarfs have incredibly high densities by earthly standards, neutron stars are even denser, cramming roughly 1.3 solar masses into a city-sized sphere. Pulsars give off radio and X-ray pulsations in lighthouse-like beams.

The discovery team, led by Yukikatsu Terada of the Institute of Physical and Chemical Research (RIKEN) in Wako, Japan, was not expecting to find a white dwarf mimicking a pulsar. Instead, the astronomers were hoping to find out if white dwarfs could accelerate charged subatomic particles to near-light speed, meaning they could be responsible for many of the cosmic rays that zip through our galaxy and occasionally strike Earth.

Some white dwarfs, including AE Aquarii, spin very rapidly and have magnetic fields millions of times stronger than Earth’s. These characteristics give them the energy to generate cosmic rays.

To find out if this is happening, Terada and his colleagues targeted AE Aquarii with Suzaku in October 2005 and October 2006. The white dwarf resides in a binary system with a normal companion star. Gas from the star spirals toward the white dwarf and heats up, giving off a glow of low-energy (soft) X-rays. But Suzaku also detected sharp pulses of hard X-rays. After analyzing the data, the team realized that the hard X-ray pulses match the white dwarf’s spin period of once every 33 seconds.

The hard X-ray pulsations are very similar to those of the pulsar in the center of the Crab Nebula. In both objects, the pulses appear to be radiated like a lighthouse beam, and a rotating magnetic field is thought to be controlling the beam. Astronomers think that the extremely powerful magnetic fields are trapping charged particles and then flinging them outward at near-light speed. When the particles interact with the magnetic field, they radiate X-rays.

"AE Aquarii seems to be a white dwarf equivalent of a pulsar," says Terada. "Since pulsars are known to be sources of cosmic rays, this means that white dwarfs should be quiet but numerous particle accelerators, contributing many of the low-energy cosmic rays in our galaxy."

Launched in 2005, Suzaku is the fifth in a series of Japanese satellites devoted to studying celestial X-ray sources. Managed by JAXA, this mission is a collaborative effort between Japanese universities and institutions and Goddard.

Adapted from materials provided by NASA/Goddard Space Flight Center.

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Supernova Surprise: Black Holes May Pull Apart, Reignite White Dwarf Stars

Daily Science Journal (Feb. 2, 2008) — A strange and violent fate awaits a white dwarf star that wanders too close to a moderately massive black hole. According to a new study, the black hole's gravitational pull on the white dwarf would cause tidal forces sufficient to disrupt the stellar remnant and reignite nuclear burning in it, giving rise to a supernova explosion with an unusual appearance. Observations of such supernovae could confirm the existence of intermediate-mass black holes, currently the subject of much debate among astronomers.

This series of images shows the interaction of a white dwarf star with a black hole. As it passes the black hole, the white dwarf becomes strongly compressed and heated (top left), triggering an explosion. Most of the stellar mass is ejected into space (the "bubble" in the upper right part of the debris in the top right image), while the rest (the cusp-like part of the image) falls toward the black hole. While the ejected matter expands rapidly, the infalling matter builds a violent, thick accretion disk around the black hole. (Credit: Image courtesy of University of California - Santa Cruz)

"Our supercomputer simulations show a peculiar supernova that would be a unique signature of an intermediate-mass black hole," said Enrico Ramirez-Ruiz, assistant professor of astronomy and astrophysics at the University of California, Santa Cruz.

Ramirez-Ruiz and his collaborators--Stephan Rosswog of Jacobs University in Bremen, Germany, and William Hix of Oak Ridge National Laboratory--used detailed computer simulations to follow the entire process of tidal disruption of a white dwarf by a black hole. Their simulations included gas dynamics, gravity, and nuclear physics, requiring weeks of computer time to simulate events that would take place in a fraction of a second.

"Every star that is not too massive ends up as a white dwarf, so they are very common. We were interested in whether tidal disruption can bring this stellar corpse to life again," said Rosswog, the first author of the paper.

A white dwarf can explode as a "type Ia" supernova if it accumulates enough mass by siphoning matter away from a companion star. When it reaches a critical mass (about 1.4 times the mass of the Sun), the white dwarf collapses and explodes. Astronomers use these type Ia supernovae as "standard candles" for cosmic distance measurements because their brightness evolves over time in a predictable manner.

The new paper* describes a distinctly different mechanism for igniting a white dwarf, in which tidal disruption by a black hole causes drastic compression of the stellar material. The white dwarf is flattened into a pancake shape aligned in the plane of its orbit around the black hole. As each section of the star is squeezed through a point of maximum compression, the extreme pressure causes a sharp increase in temperatures, which triggers explosive burning.

The explosion ejects more than half of the debris from the disrupted star, while the rest of the stellar material falls into the black hole. The infalling material forms a luminous accretion disk that emits x-rays and should be detectable by the Chandra X-ray Observatory, the researchers said.

"This is a new mechanism for ignition of a white dwarf that results in a very different type of supernova than the standard type Ia, and it is followed by an x-ray source," Ramirez-Ruiz said.

He estimated that this type of event would occur about 100 times less frequently than the standard type Ia supernovae, but should be detectable by future surveys designed to observe large numbers of supernovae. The Large Synoptic Survey Telescope (LSST), planned for completion in 2013, is expected to discover hundreds of thousands of type Ia supernovae per year.

"These exotic creatures will start showing up in the data from the LSST," Ramirez-Ruiz said. "We want to predict the light curves so we can look for them in the survey data."

The mechanism described in the paper requires a black hole that is neither too small nor too big. Such intermediate-mass black holes (500 to 1,000 times the mass of the Sun) may reside in some globular star clusters, but there is much less evidence for their existence than there is for the relatively small stellar black holes (tens of times the mass of the Sun) or for supermassive black holes (a few million times the mass of the Sun), found at the centers of galaxies.

The new paper describes in detail the disruption of a white dwarf with two-tenths the mass of the Sun by a black hole 1,000 times the mass of the Sun. The researchers also found that they can vary the mass of the white dwarf and still get the same outcome--tidal disruption and ignition of the white dwarf.

"We can ignite the whole mass range of white dwarfs if they get close enough to the black hole," Rosswog said.

*A paper describing their results has been accepted for publication in Astrophysical Journal Letters, and a preprint is currently available online.

This research was supported by the Department of Energy's Program for Scientific Discovery through Advanced Computing.

Adapted from materials provided by University of California - Santa Cruz.

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Nova Finding Challenges Thinking On Powerful Stellar Explosions

Daily Science Journal (Feb. 2, 2008) — First results from a new NASA-funded scientific instrument at the W. M. Keck Observatory in Hawaii are helping scientists overturn long-standing assumptions about powerful explosions called novae and have produced the first unified model for a nearby nova called RS Ophiuchi.

This artist rendering depicts the RS Ophiuchi binary system shortly after the white dwarf (right) has exploded as a nova. The other star is a red giant. Note the spiral dust lanes. (Credit: Casey Reed; Courtesy of NASA/Goddard Space Flight Center)

"We were getting ready for a routine engineering run when all of a sudden the nova went off. It was very bright and easy to observe, so we took this opportunity and turned it into gold," says team member Marc Kuchner of NASA's Goddard Space Flight Center in Greenbelt, Md.

Kuchner and his colleagues used the "nulling" mode of the Keck Interferometer, which is part of the NASA-funded Keck Interferometer. This state-of-the-art instrument combines starlight using two 10-meter (33 feet) telescopes. In the nulling mode, the interferometer suppresses the blinding light of a star so researchers can study the surrounding environment. The instrument helps researchers observe very faint objects near bright sources and produces ten times more resolving power than a single Keck telescope working alone. It is the only instrument of its kind in operation.

The Keck Nuller was undergoing tests on February 12, 2006, when a nova flared up in the constellation Ophiuchus. The system, known as RS Ophiuchi, consists of a white dwarf and a red giant. The red giant is gradually shedding its massive gaseous outer layers, and the white dwarf is sweeping up much of this wind, growing in mass over time. As the matter builds up on the white dwarf's surface it eventually reaches a critical temperature that ignites a thermonuclear explosion that causes the system to brighten 600-fold. RS Ophiuchi was previously seen to blow its stack in 1898, 1933, 1958, 1967, and 1985, so astronomers were eagerly anticipating the 2006 eruption.

Just 3.8 days after the nova was detected, the group observed the explosion with the Keck Nuller. The team set the instrument to cancel out the nova's light, allowing the group to see the much fainter surrounding material. The group next adjusted the nuller to observe the extremely bright blast zone.

The instrument's versatility was key to a surprising discovery. The nuller saw no dust in the bright zone, presumably because the nova's blast wave vaporized dust particles. But farther from the white dwarf, at distances starting around 20 times the Earth-Sun distance, the nuller recorded the spectral signature of silicate dust. The blast wave had not yet reached this zone, so the dust must have pre-dated the explosion.

"This flies in the face of what we expected. Astronomers had previously thought that nova explosions actually create dust," says Richard Barry of NASA Goddard, lead author of a paper on the Keck observations that will be published in the Astrophysical Journal.

The team thinks the dust is created as the white dwarf plows through the red giant's wind, creating a pinwheel pattern of higher-density regions that is reminiscent of galaxy spiral arms. Inside these spiral arms, atoms reach low enough temperatures and high enough densities to allow atoms to stick together to form dust particles. The nova's blast wave has since destroyed RS Ophiuchi's pinwheel pattern, but it should re-form over the next few years, and future Spitzer Space Telescope observations could see it.

Most studies of RS Ophiuchi have relied on spectroscopic models, but those methods have not been able to distinguish various nova components with as much detail as the interferometer. The Keck Nuller measured one component of the RS Ophiuchi system to an accuracy of just 4 milliarcseconds, or about the size of a basketball seen 7,500 miles away.

Barry is also coauthor of a paper based on Spitzer observations of RS Ophiuchi. This paper reports independent evidence for silicate dust that predates the 2006 explosion.

"The RS Ophiuchi observations are just a small taste of the power and potential we expect from the Keck Nuller," says coauthor William Danchi of NASA Goddard. "But ultimately we want to launch a nulling interferometer into space to image extrasolar planets. These Keck results are a technological and scientific pathfinder toward that future."

Adapted from materials provided by NASA/Goddard Space Flight Center.

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New Discovery On Magnetic Reconnection To Impact Future Space Missions ; Magnetic Fields Get Reconnected In Turbulent Plasma Too, Cluster Reveals

Daily Science Journal (Jan. 31, 2008) — ESA’s Cluster mission has, for the first time, observed the extent of the region that triggers magnetic reconnection, and it is much larger than previously thought. This gives future space missions a much better chance of studying it.

In a plasma (a gas of charged particles), during magnetic reconnection, magnetic field lines of opposite direction break and then reconnect, forming an X-line magnetic topology. The newly reconnected field lines accelerate the plasma away from the X-line. (Credit: Center for Visual computing, Univ. of California Riverside)

Space is filled with plasma (a gas composed of ions and electrons, globally neutral) and is threaded by magnetic fields. These magnetic fields store energy which can be released explosively, in a process called magnetic reconnection.

This process plays a key role in numerous astrophysical phenomena: star formation, solar flares and intense aurorae, to name a few. On Earth, magnetic reconnection prevents the efficient production of electricity in controlled fusion reactors, potential sources of electricity for the future.

Schematic of magnetic field lines during reconnection

At the heart of magnetic reconnection is the ‘electron diffusion region’, where reconnection is thought to be triggered. Here, a kink in newly-reconnected magnetic field lines produces large-scale high-velocity jets of plasma.

“Understanding the structure of the diffusion region and its role in controlling the rate at which magnetic energy is converted into particle energy remains a key scientific challenge,” says Dr Michael Shay, University of Delaware, USA.

Until recently, theoretical scientists believed that the electron diffusion region was relatively tiny (width about 2 km, length about 10 km). In the vastness of space, the chance of a spacecraft encountering this region would therefore be exceedingly small.

With increased computational power, simulations showed electron diffusion regions that were a lot more elongated than those seen earlier. It was not possible to judge whether the new finding was real because the length of the region increased with more powerful simulations. Nor it was known whether such a layer would be stable in the real, 3D world.

Comparison between observations and simulation

On 14 January 2003, the four Cluster satellites were crossing the magnetosheath, a turbulent plasma region located just outside Earth’s magnetosphere, when they encountered an electron diffusion region. The length of the observed region measured 3000 km, 300 times longer than the earlier theoretical expectations and four times longer than seen in recent simulations. Nevertheless, the observations strongly support new simulations.

“These Cluster observations are very significant since they are the first measurements of the length of the electron diffusion region in the space environment. The finding drastically changes the way we understand the physics of reconnection,” noted Dr James Drake, University of Maryland, USA.

“This discovery of a large electron diffusion region gives future ESA and NASA missions a much better chance to study it,” said Tai Phan at the University of California at Berkeley, USA, lead author of the paper on the findings.

Magnetic reconnection simulation

Cluster was able to detect the region based on its high-resolution magnetic field, electric field and ion measurements. But to understand the fundamental physics of the electron diffusion region responsible for reconnection, higher time resolution measurements are needed to resolve the layer.

The four spacecraft of NASA’s Magnetospheric Multi-Scale mission, planned for launch in 2014, are being designed for such measurements. Cross-scale, a mission under study at ESA in collaboration with other space agencies, would use 12 spacecraft to probe the diffusion region, whilst simultaneously measuring the consequences of energy released by reconnection in the surrounding environment.

“With the higher probability of encountering the electron diffusion region, we can be confident that future missions will be able to fully understand magnetic reconnection,” said Dr Philippe Escoubet, ESA’s Cluster and Double Star Project Scientist and Cross-scale Study Scientist.

The findings appear in, ‘Evidence for an elongated (> 60 ion skin depths) electron diffusion region during fast magnetic reconnection,’ by T. Phan, J. Drake, M. Shay, F. Mozer and J. Eastwood, published in the Physical Review Letters, on 21 December 2007.

Adapted from materials provided by European Space Agency.

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Magnetic Fields Get Reconnected In Turbulent Plasma Too, Cluster Reveals

Using measurements of the four ESA's Cluster satellites, a study published in Nature Physics shows pioneering experimental evidence of magnetic reconnection also in turbulent 'plasma' around Earth.

This image provides a model of magnetic fields at the Sun's surface using SOHO data, showing irregular magnetic fields (the 'magnetic carpet') in the solar corona (top layer of the Sun's atmosphere). Small-scale current sheets are likely to form in such turbulent environment and reconnection may occur in similar fashion as in Earth's magnetosheath. This can be relevant to a better understanding of the heating of solar corona. (Credit: Stanford-Lockheed Inst. for Space Research/NASA GSFC)

Magnetic reconnection – a phenomenon by which magnetic fields lines get interconnected and reconfigure themselves - is a universal process in space that plays a key role in various astrophysical phenomena such as star formation, solar explosions or the entry of solar material within the Earth's environment. Reconnection has been observed at large-scale boundaries between different plasma environments such as the boundary between Earth and interplanetary space. Plasma is a gas composed of charged particles.

An irregular behaviour of particle flows and magnetic fields causes plasma turbulence within which many small-scale boundaries can form, where reconnection has been predicted via modelling. However, thanks to Cluster this was the first time that this could be directly observed, opening up new perspectives to help us better understand the behaviour of turbulent plasma.

Our first line of defence against the incessant flow of solar particles, the Earth's magnetic field deflects most of this material around the Earth's magnetosphere. This is marked by a boundary layer called the magnetopause. As for any other planet which has a planetary magnetic field (for example Jupiter and Saturn), solar wind is decelerated from supersonic to subsonic speeds by a shock wave (called the 'bow shock') located in front of the magnetopause. The region between the bow shock and the magnetopause is called the magnetosheath.

One of the most turbulent environments in the near-Earth space, the terrestrial magnetosheath is an accessible laboratory to study in-situ turbulence, unlike the solar atmosphere or accretion disks. Characterising the properties of the magnetic turbulence in this region is of prime importance to understand its role in fundamental processes such as energy dissipation and particle acceleration.

Observing reconnection at small-scale boundaries in space requires simultaneous measurements by at least four spacecraft flying in close formation. With an inter-spacecraft distance of only 100 kilometres, on 27 March 2002 the four Cluster satellites observed reconnection within a very thin current 'sheet' embedded in the turbulent plasma with a typical size of about 100 kilometres.

A challenge for the instruments onboard, the observations show that the turbulent plasma is accelerated and heated during the reconnection process. This newly observed type of small-scale reconnection seems also to be associated with the acceleration of particles to energies much higher than their average which could explain, in part, the creation of high energy particles by the Sun.

To quote Alessandro Retinò, lead author of this study and PhD student at the Swedish Institute of Space Physics, Uppsala, Sweden, "we found reconnection in one single current sheet, so that in such an environment of irregular magnetic fields one may think that reconnection is sporadic, but this is not the case. For this particular magnetosheath crossing, a very large number of other thin current sheets was found where reconnection is very likely to occur, a subject currently under investigation by our team."

This discovery of reconnection in turbulent plasma has significant implications for the study of laboratory and astrophysical plasmas, where both turbulence and reconnection develop and thus where turbulent reconnection is very likely to occur. Possible applications range from the dissipation of magnetic energy in fusion devices on Earth to the understanding of the acceleration of high energy particles in solar explosions called solar flares.

"Magnetic reconnection, turbulence and shocks are three fundamental ingredients of the plasma Universe," says Philippe Escoubet Cluster and Double Star project scientist at ESA. "The detailed understanding of these key processes and their associated multi-scale physics is a challenge for the future of space physics. One of the lessons learned from Cluster is the need for new space missions equipped with instruments of higher sensitivity and better time resolution together with a larger number of satellites."

Adapted from materials provided by European Space Agency.

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Astronomers Discover How White Dwarf Stars Get Their 'Kicks'

Daily Science Journal (Dec. 5, 2007) — University of British Columbia astronomer Harvey Richer and UBC graduate student Saul Davis have discovered that white dwarf stars are born with a natal kick, explaining why these smoldering embers of Sun-like stars are found on the edge rather than at the centre of globular star clusters.

These images show young and old white dwarf stars — the burned-out relics of normal stars — in the ancient globular star cluster NGC 6397. The image at left shows the dense swarm of hundreds of thousands of stars that make up the globular cluster. The image at top, right reveals young white dwarfs less than 800 million years old and older white dwarfs between 1.4 and 3.5 billion years old. The blue squares pinpoint the young white dwarfs; the red circles outline the older white dwarfs. (Credit: D. Verschatse (Antilhue Observatory, Chile), NASA, ESA, and H. Richer (University of British Columbia))

White dwarfs represent the third major stage of a star's evolution. Like the Sun, each star begins its life with a long stable state where nuclear reactions take place in the core supplying the energy. After the core fuel is depleted, it swells up and turns into a huge red giant. Later, the red giant ejects its outer atmosphere and its core becomes a white dwarf that slowly cools over time and radiates its stored thermal heat into space.

Using NASA's Hubble telescope, Richer and his team looked at the position of white dwarfs in NGC 6397, one of the globular star clusters closest to Earth. Globular clusters are dense swarms of hundreds of thousands of stars. About 150 of these clusters exist in the Milky Way, each containing between 100,000 and one million stars.

"The distribution of young white dwarfs is the exact opposite of what we expected," says Prof. Richer, whose study will appear in the Monthly Notices of the Royal Astronomical Society Letters in January 2008.

Richer explains that globular clusters sort out stars according to their mass, governed by a gravitational billiard-ball game among stars. Heavier stars slow down and sink to the cluster's core, while lighter stars pick up speed and move across the cluster to its outskirts. The team found that the older white dwarfs were behaving as expected; they were scattered throughout the cluster according to weight.

"Newly-minted white dwarfs should be near the center, but they are not," says Richer. "Our idea is that when these white dwarfs were born, they were given a small kick of 7,000 to 11,000 miles an hour (three to five kilometers a second), which rocketed them to the outer reaches of the cluster."

Using computer simulations, Richer and his team showed that when white dwarfs were born, their own mass acts like "rocket fuel" propelling them forward.

"If more of this mass is ejected in one direction, it could propel the emerging white dwarf through space, just as exhaust from a rocket engine thrusts it from the launch pad," says Richer.

The researchers studied 22 young white dwarfs up to about 800 million years old and 62 older white dwarfs between 1.4 and 3.5 billion years old. They distinguished the younger from the older white dwarfs based on their color and brightness. The younger ones are hotter, and therefore bluer and brighter than the older ones.

Study co-authors are: I. King, University of Washington; J.Anderson, Space Telescope Science Institute; J. Coffey, UBC, G. Fahlman, National Research Council of Canada's Herzberg Institute of Astrophysics; J Hurley. Swinburne, University of Technology; and J. Kalirai, University of California, Santa Cruz.

Adapted from materials provided by University of British Columbia, via EurekAlert!, a service of AAAS.

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Astronomers Discover Stars With Carbon Atmospheres

Daily Science Journal (Nov. 22, 2007) — Astronomers have discovered white dwarf stars with pure carbon atmospheres. The discovery could offer a unique view into the hearts of dying stars.

Artists' concept of the surface of the white dwarf star H1504+65, believed to have somehow expelled all its hydrogen and all but a very small trace of its helium, leaving an essentially bare stellar nucleus with a surface of 50 percent oxygen and 50 percent carbon. When this star cools, it may have a carbon atmosphere, like the stars newly found by University of Arizona, Canadian and French astronomers. (Credit: Illustration credit: M.S. Sliwinski and L. I. Slivinska of Lunarismaar, Copyright photo by Sliwinski, M.S. and Sliwinska, L.I.)

These stars possibly evolved in a sequence astronomers didn't know before. They may have evolved from stars that are not quite massive enough to explode as supernovae but are just on the borderline. All but the most massive two or three percent of stars eventually die as white dwarfs rather than explode as supernovae.

When a star burns helium, it leaves "ashes" of carbon and oxygen. When its nuclear fuel is exhausted, the star then dies as a white dwarf, which is an extremely dense object that packs the mass of our sun into an object about the size of Earth. Astronomers believe that most white dwarf stars have a core made of carbon and oxygen which is hidden from view by a surrounding atmosphere of hydrogen or helium.

They didn't expect stars with carbon atmospheres.

"We've found stars with no detectable traces of helium and hydrogen in their atmospheres," said University of Arizona Steward Observatory astronomer Patrick Dufour. "We might actually be observing directly a bare stellar core. We possibly have a window on what used to be the star's nuclear furnace and are seeing the ashes of the nuclear reaction that once took place."

Dufour, UA astronomy Professor James Liebert and their colleagues at the Université de Montréal and Paris Observatory published the results in the Nov. 22 issue of Nature.

The stars were discovered among 10,000 new white dwarf stars found in the Sloan Digital Sky Survey. The survey, known as the SDSS, found about four times as many white dwarf stars previously known.

Liebert identified a few dozens of the newfound white dwarfs as "DQ" white dwarfs in 2003. When observed in optical light, DQ stars appear to be mostly helium and carbon. Astronomers believe that convection in the helium zone dredges up carbon from the star's carbon-oxygen core.

Dufour developed a model to analyze the atmospheres of DQ stars as part of his doctoral research at the Université de Montréal. His model simulated cool DQ stars, stars at temperatures between 5,000 degrees and 12,000 degrees Kelvin. For reference, our sun's surface temperature is around 5,780 degrees Kelvin.

When Dufour joined Steward Observatory in January, he updated his code to analyze hotter stars, stars as hot as 24,000 degrees Kelvin.

"When I first started modeling the atmospheres of these hotter DQ stars, my first thought was that these are helium-rich stars with traces of carbon, just like the cooler ones," Dufour said. "But as I started analyzing the stars with the higher temperature model, I realized that even if I increased the carbon abundance, the model still didn't agree with the SDSS data," Dufour said.

In May 2007, "out of pure desperation, I decided to try modeling a pure-carbon atmosphere. It worked," Dufour said. "I found that if I calculated a pure carbon atmosphere model, it reproduces the spectra exactly as observed. No one had calculated a pure carbon atmosphere model before. No one believed that it existed. We were surprised and excited."

Dufour and his colleagues have identified eight carbon-dominated atmosphere white dwarf stars among about 200 DQ stars they've checked in the Sloan data so far.

The great mystery is why these carbon-atmosphere stars are found only between about 18,000 degrees and 23,000 degrees Kelvin. "These stars are too hot to be explained by the standard convective dredge-up scenario, so there must be another explanation," Dufour said.

Dufour and Liebert say they these stars might have evolved from a star like the unique, much hotter star called H1504+65 that Pennsylvania State University astronomer John A. Nousek, Liebert and others reported in 1986. If so, carbon-atmosphere stars represent a previously unknown sequence of stellar evolution.

H1504+65 is a very massive star at 200,000 degrees Kelvin.

Astronomers currently believe this star somehow violently expelled all its hydrogen and all but a very small trace of its helium, leaving an essentially bare stellar nucleus with a surface of 50 percent carbon and 50 percent oxygen.

"We think that when a star like H1504+65 cools, it eventually becomes like the pure-carbon stars," Dufour said. As the massive star cools, gravity separates carbon, oxygen and trace helium. Above 25,000 degrees Kelvin, the trace helium rises to the top, forming a thin layer above the much more massive carbon envelope, effectively disguising the star as a helium-atmosphere white dwarf, Dufour and Liebert said.

But between 18,000 and 23,000 degrees Kelvin, convection in the carbon zone probably dilutes the thin helium layer. At these temperatures, oxygen, which is heavier than carbon, has probably sunk too deep to be dredged to the surface.

Dufour and his colleagues say that models of stars nine to 11 solar masses might explain their peculiar carbon stars.

Astronomers predicted in 1999 that stars nine or 10 times as massive as our sun would become white dwarfs with oxygen-magnesium-neon cores and mostly carbon-oxygen atmospheres. More massive stars explode as supernovae.

But scientists aren't sure where the dividing line is, whether stars eight, nine, 10 or 11 times as massive as our sun are required to create supernovae.

"We don't know if these carbon atmosphere stars are the result of nine-or-10 solar mass star evolution, which is a key question," Liebert said.

The UA astronomers plan making new observations of the carbon atmosphere stars at the 6.5-meter MMT Observatory on Mount Hopkins, Ariz., in December to better pinpoint their masses. The observations could help define the mass limit for stars dying as white dwarfs or dying as supernovae, Dufour said.

Adapted from materials provided by University of Arizona.

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Double Star TC-1 Completes Its Mission ; Details Of Solar Particles Penetrating The Earth's Environment Revealed

Daily Science Journal (Oct. 19, 2007) — TC-1, one of the two satellites of the CNSA/ESA Double Star mission, was decommissioned on 14 October as its designed orbit lifetime came to an end. The satellite re-entered Earth’s atmosphere and turned to dust during its descent.

Flying in formation around the Earth, they relay detailed information about how solar wind affects our planet in 3D. The satellites are called Rumba, Salsa, Samba and Tango. (Credit: ESA)

Along with its twin TC-2, TC-1 is the first satellite built and operated by the Chinese National Space Administration (CNSA) in cooperation with ESA. Along with its twin and the four Cluster satellites, TC-1 has helped accomplish much during its lifetime.

The four years during which Double Star was operational brought in new perspectives concerning the boundaries of the magnetosphere and the fundamental processes that are playing a role in the transport of mass, momentum and energy into the magnetosphere. Thanks to the measurements of TC-1, there was a chance to observe the evolution of structures and physical processes at small scales with Cluster, and then on large scales with Double Star.

Here we list some of the most interesting results where TC-1 played a crucial role.

Space is fizzy

Above our heads, at the bow shock, where the Earth’s magnetic field meets the constant stream of gas from the Sun, thousands of bubbles of superheated gas, or ion density holes, are constantly growing and popping. These bubbles were discovered by Cluster and Double Star together, and the discovery allowed scientists to better understand the interaction between the solar wind and the Earth’s magnetic field.

Celestial chorus further away

Chorus emissions are waves naturally generated in space close to the magnetic equator. They play an important role in creating killer electrons that can damage solar panels and electronic equipments of satellites and are a hazard for astronauts. It was found that these waves are created further away from Earth during high geomagnetic activity. This information is crucial to be able to forecast their impact.

Oscillations of Earth’s natural cloak of magnetism

The four Cluster satellites and TC-1 unexpectedly found themselves engulfed by waves of electrical and magnetic energy as they travelled through Earth’s night-time shadow. Something had set the tail of Earth’s natural cloak of magnetism oscillating, like waves created by a boat travelling across a lake. The data collected gave scientists an important clue to the effects of space weather on Earth’s magnetic field.

"Double Star has demonstrated mutual benefit and fostered scientific cooperation in space research between China and Europe. But there is still much more to come as the full, high-resolution data archive becomes available," says Philippe Escoubet, ESA’s Cluster and Double Star Project Scientist.

Adapted from materials provided by European Space Agency.

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Details Of Solar Particles Penetrating The Earth's Environment Revealed

Co-ordinated efforts by China/ESA’s Double Star and ESA’s Cluster spacecraft have allowed scientists to zero in on an area where energetic particles from the Sun are blasting their way through the Earth’s magnetic shield. Solar material penetrating the Earth's magnetic shield can represent a hazard to both astronauts and satellites.

This sketch shows the orbits of the Cluster and Double Star (TC-1) spacecraft on 8 May 2004, when the five satellites observed magnetic channels created by the merging of the Sun and the Earth's magnetic fields. Such events, called 'Flux Transfer Events,' allow solar particles to break through the Earth's magnetic shield and penetrate the Earth's environment. (Image courtesy of European Space Agency)

On 8 May 2004, one of the two Double Star satellites (TC-1) and all four Cluster spacecraft found themselves in the firing line. For about 6 hours, the Cluster spacecraft were buffeted every 8 minutes by intense flows of electrically charged particles released by the Sun. The Double Star TC-1 spacecraft had it even rougher, being blasted every four minutes for eight hours.

During such events, magnetic channels created by the merging of the Sun and the Earth’s magnetic fields allow solar particles to break through the Earth’s magnetic shield and penetrate the Earth’s environment. Physicists call the occurrence of these magnetic channels Flux Transfer Events. Each magnetic channel appears like a curve shaped tube that can be anything from 5000 to 25000 kilometres in diameter. One end of the magnetic flux tube is connected to Earth while the other end is connected to the solar wind.

The basic physical mechanism responsible for the occurrence of flux transfer events is called magnetic reconnection. In the 1950s, space physicists believed that magnetic reconnection let solar particles break through at a steady rate. That view changed in the late 1970s, when several studies showed that the magnetic reconnection could also be intermittent and take place in pulses, lasting a few minutes. Each pulse produces a magnetic flux tube (a Flux Transfer Event).

On 8 May 2004, these magnetic flux tubes swept over Cluster and Double Star again and again. As the Cluster and Double Star data clearly showed, the same location underwent magnetic reconnection several times, creating new successive magnetic flux tubes to channel more charged particles towards the Earth. The observations stopped probably because the spacecraft moved out of range and not because the reconnection region weakened in any way.

The data from the five spacecraft allowed scientists led by Aurélie Marchaudon of the Laboratoire de Physique et Chimie de l’Environnement, Centre Nationale de la Recherche Scientifique (CNRS) and Université d’Orléans, Orléans, France to triangulate the location of the magnetic reconnection region, and to deduce its size. They found that the reconnection site was located on the daylight west side of the Earth’s magnetic shield and was around 25000 kilometres across. A computer simulation of the event, conducted by Jean Berchem of the University of California Los Angeles (UCLA) and his team, confirmed the possibility of magnetic reconnection occurring at that location.

Although intermittent reconnection has been observed in the past, this was one of the longest series of continuous observations ever taken of a magnetic reconnection region in the Earth’s magnetosphere. Perhaps most surprising is that 8 May 2004 was just relatively a normal day for the Earth’s magnetic field. There were no large magnetic storms on Earth, or spectacular aurorae to fill the night sky. However, Cluster and Double Star revealed that energetic particles from the Sun were blasting their way through the Earth’s magnetic shield and penetrating the Earth’s environment.

Each day, Cluster and Double Star return more observations that allow scientist to understand the invisible magnetic turbulence high above our heads.

Adapted from materials provided by European Space Agency.

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Cluster And Double Star Uncover More On Bright Aurorae

Daily Science Journal (Sep. 12, 2007) — Cluster data has helped provide scientists with a new view of magnetospheric processes, challenging existing theories about magnetic substorms that cause aurorae and perturbations in GPS signals.

The diagram shows all 11 spacecraft of the Cluster, Double Star and THEMIS missions orbiting the Earth. All three missions are studying the environment of Earth's magnetosphere. (Credit: ESA)

The onset of magnetic substorms that originate in Earth’s magnetosphere has been explained by two competing models: current disruption and near-earth reconnection. Current beliefs have been challenged using data from ESA’s Cluster satellites, and CNSA’s Double Star, a mission with ESA participation. Recent research suggests a third type of substorm onset.

Magnetic substorms often cause bright and colourful aurorae at high latitudes, in places such as Scandinavia or Canada. These aurorae are caused by energetic electrons that spiral down Earth’s magnetic field lines and collide with atmospheric atoms at an altitude of about 100 km. The energetic electrons come from the magnetotail, located on the nightside of Earth where the solar wind stretches Earth’s magnetic field lines into a long tail.

At the centre of the magnetotail is a denser region known as the plasma sheet. Plasma is a gas composed of ions and electrons which is electrically neutral. It is spread over large distances in space and guided by the action of magnetic and electric fields. A substorm induces violent changes in the plasma sheet. It energises ions and electrons and hurls them Earthward. The substorm itself can occur as a series or in isolation.

Apart from producing the beautiful show of light, substorms also excite a large portion of Earth's ionosphere, perturbing the reception of GPS signals and communication between Earth and orbiting satellites. Despite decades of space research, the mechanism causing the onset of substorms remains a mystery.

There are three events associated with the onset of a substorm: auroral brightening, current disruption, and magnetic reconnection.

Auroral brightening is a sudden change of the aurora from light grey to very bright and colourful auroras at an altitude of about 100 km, visible from ground. Current disruption occurs at a height of roughly 60 000 km on the nightside or at a sixth of the distance to the Moon and is associated with turbulent fluctuations in the magnetic field.

Magnetic reconnection is the process whereby magnetic field lines from different magnetic domains collide and reconnect, heating and accelerating plasma. It occurs at around a third of the distance to the Moon or at a height of 120 000 km, in a thin plane close to the magnetic equator of the magnetotail.

The difference between the two existing theories on magnetic substorms is that they differ on the order in which the events take place.

Prof Sergeev (St Petersburg State University, Russia) and colleagues from Europe, the USA and China studied three consecutive substorm onsets, from data collected on board Cluster and Double Star on 26 September 2005. For the first time, data indicate that the current disruption process and magnetic reconnection can coincide in space and time showing, possibly, two sides of the same process.

They also found that in this case, magnetic reconnection occurred closer to the Earth than usual, almost co-located with the current disruption process, between 60 000 and 90 000 km. Related localised auroral brightenings were captured few tens of seconds later by an ultra violet imager onboard the NASA’s IMAGE satellite.

“Cluster’s multipoint measurements and the spatial coverage possible together with Double Star have been instrumental in making these unique observations possible,” commented Sergeev.

In February 2007, NASA launched THEMIS, a five-satellite mission dedicated to the study of the onset of substorms. "With the many scientific satellites in orbit, we have a never-before opportunity to study the global solar-magnetospheric environment and the physical processes involved," said Philippe Escoubet, Cluster and Double Star project scientist of the European Space Agency.

The results appear in ‘Observation of repeated intense near-Earth reconnection on closed field lines with Cluster, Double Star and other spacecraft’ by V. Sergeev, V. Semenov, M. Kubyshkina, V. Ivanova, W. Baumjohann, R. Nakamura, T. Penz, A. Runov, T. L. Zhang, K. Glassmeier, V. Angelopoulos, H. Frey, J. Sauvaud, P. Daly, J. Cao, H. Singer, and E. Lucek. The paper is published in the 20 January 2007 issue of the Geophysical Research Letters.

Adapted from materials provided by European Space Agency.

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NASA'S Swift Sees Double Supernova In Galaxy

Daily Science Journal (Jun. 27, 2007) — In just the past six weeks, two supernovae have flared up in an obscure galaxy in the constellation Hercules. Never before have astronomers observed two of these powerful stellar explosions occurring in the same galaxy so close together in time.

Supernova 2007ck (left) is a Type II event, and Supernova 2007co (right) is a Type Ia event. The image is a combination of red, green, and blue pictures taken on June 9 and 12 by the Ultraviolet/Optical Telescope on NASA’s Swift satellite, which was designed primarily to study another type of stellar explosion – gamma ray bursts. (Credit: Stefan Immler NASA/GSFC, Swift Science Team.)

The galaxy, known as MCG +05-43-16, is 380 million light-years from Earth. Until this year, astronomers had never sighted a supernova popping off in this stellar congregation. A supernova is an extremely energetic and life-ending explosion of a star.

Making the event even more unusual is the fact that the two supernovae belong to different types. Supernova 2007ck is a Type II event – which is triggered when the core of a massive star runs out of nuclear fuel and collapses gravitationally, producing a shock wave that blows the star to smithereens. Supernova 2007ck was first observed on May 19.

In contrast, Supernova 2007co is a Type Ia event, which occurs when a white dwarf star accretes so much material from a binary companion star that it blows up like a giant thermonuclear bomb. It was discovered on June 4, 2007. A white dwarf is the exposed core of a star after it has ejected its atmosphere; it’s approximately the size of Earth but with the mass of our Sun.

"Most galaxies have a supernova every 25 to 100 years, so it’s remarkable to have a galaxy with two supernovae discovered just 16 days apart," says Stefan Immler of NASA’s Goddard Space Flight Center. In 2006 Immler used NASA’s Swift satellite to image two supernovae in the elliptical galaxy NGC 1316, but both of those explosions were Type Ia events, and they were discovered six months apart.

The simultaneous appearance of two supernovae in one galaxy is an extremely rare occurrence, but it’s merely a coincidence and does not imply anything unusual about MCG +05-43-16. Because the two supernovae are tens of thousands of light-years from each other, and because light travels at a finite speed, astronomers in the galaxy itself, or in a different galaxy, might record the two supernovae exploding thousands of years apart.

Adapted from materials provided by NASA/Goddard Space Flight Center.

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Diamonds Unlikely In Gas Giants Like Uranus

Daily Science Journal (Jul. 16, 2007) — A new study finds that diamonds probably don't crystallize in the atmospheres of planets such as Uranus and Neptune. The conclusion is contrary to recent speculation that small diamonds would spontaneously form in carbon rich layers of the gas giant planets. White dwarf stars, according to the study, are veritable diamond factories.

A simulated snapshot of crystallizing carbon atoms under Uranus-like conditions. (Credit: L. M. Ghiringhelli, C. Valeriani, E. J. Meijer and D. Frenkel, Physical Review Letters)

Physicists at the Universtiet van Amsterdam and the FOM Institute for Atomic and Molecular Physics in the Netherlands performed a numerical analysis showing that at the temperatures and pressures in gas giant planets like Uranus, arrangements of carbon atoms would be much more suitable for creating tiny bits of graphite rather than diamond.

In white dwarfs, on the other hand, the simulation shows that the conditions would cause the carbon atoms to line up in configurations that are much more amenable for diamond crystallization. The conclusion is consistent with the 2004 discovery of a cooling white dwarf star that appears to have a solid diamond core 4000 kilometers across.

Although diamond formation in the atmospheres of gas giants is not strictly impossible, the Dutch physicists say that the odds are exceedingly slim that a diamond could have formed under the conditions that exist in Uranus in the entire lifetime of the universe.

Adapted from materials provided by American Physical Society, via EurekAlert!, a service of AAAS.

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Stellar Tiramisu: Astronomers Find Hints of Distant Planetary Pollution

Daily Science Journal (Jul. 10, 2007) — Looking at the chemical composition of stars that host planets, astronomers have found that while dwarf stars often show iron enrichment on their surface, giant stars do not. The astronomers think that the planetary debris falling onto the outer layer of the star produces a detectable effect in a dwarf star, but this pollution is diluted by the giant star and mixed into its interior.

Artist's impression of the structure of a solar-like star and a red giant. The two images are not to scale - the scale is given in the lower right corner. (Credit: Copyright ESO)

"It is a little bit like a Tiramisu or a Capuccino," says Luca Pasquini from ESO, lead-author of the paper reporting the results. "There is cocoa powder only on the top!'

Just a few years after the discovery of the first exoplanet it became evident that planets are preferentially found around stars that are enriched in iron. Planet-hosting stars are on average almost twice as rich in metals than their counterparts with no planetary system.

The immediate question is whether this richness in metals enhances planet formation, or whether it is caused by the presence of planets. The classic chicken and egg problem. In the first case, the stars would be metal-rich down to their centre. In the second case, debris from the planetary system would have polluted the star and only the external layers would be affected by this pollution.

When observing stars and taking spectra, astronomers indeed only see the outer layers and can't make sure the whole star has the same composition. When planetary debris fall onto a star, the material will stay in the outer parts, polluting it and leaving traces in the spectra taken.

A team of astronomers has decided to tackle this question by looking at a different kind of stars: red giants. These are stars that, as will the Sun in several billion years, have exhausted the hydrogen in their core. As a result, they have puffed up, becoming much larger and cooler.

Looking at the distribution of metals in fourteen planet-hosting giants, the astronomers found that their distribution was rather different from normal planet-hosting stars.

"We find that evolved stars are not enriched in metals, even when hosting planets," says Pasquini. "Thus, the anomalies found in planet-hosting stars seem to disappear when they get older and puff up!"

Looking at the various options, the astronomers conclude that the most likely explanation lies in the difference in the structure between red giants and solar-like stars: the size of the convective zone, the region where all the gas is completely mixed. In the Sun, this convective zone comprises only 2% of the star's mass. But in red giants, the convective zone is huge, encompassing 35 times more mass. The polluting material would thus be 35 times more diluted in a red giant than in a solar-like star.

"Although the interpretation of the data is not straightforward, the simplest explanation is that solar-like stars appear metal-rich because of the pollution of their atmospheres," says co-author Artie Hatzes, Director of the Thüringer Landessternwarte Tautenburg (Germany) where some of the data were obtained.

When the star was still surrounded by a proto-planetary disc, material enriched in more heavy elements would fall onto the star, thereby polluting its surface. The metal excess produced by this pollution, while visible in the thin atmospheres of solar-like stars, is completely diluted in the extended, massive atmospheres of the giants.

"Evolved stars hint to an external origin of enhanced metallicity in planet-hosting stars", by L. Pasquini et al. To appear in Astronomy and Astrophysics.

The team is composed of L. Pasquini and M.P. Döllinger (ESO), A. Weiss (Max-Planck-Institut für Astrophysik, Garching, Germany), L. Girardi (INAF-Osservatorio Astronomico di Padova, Italy), C. Chavero (Instituto de Astrofisica de Canarias, Tenerife, Spain, and Observatorio Nacional/MCT, Rio de Janeiro, Brasil), A. P. Hatzes (Thüringer Landessternwarte Tautenburg, Germany), L. da Silva (Observatorio Nacional/MCT, Rio de Janeiro, Brasil), and J. Setiawan (Max Planck Institute für Astronomie, Heidelberg, Germany).

The data have been partially collected at ESO, and partially at the 2-m telescope of the Thüringer Landessternwarte Tautenburg (TLS).

Adapted from materials provided by European Southern Observatory.

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Astronomers Find First Habitable Earth-like Planet

Daily Science Journal (Jun. 24, 2007) — Astronomers have discovered the most Earth-like planet outside our Solar System to date, an exoplanet with a radius only 50% larger than the Earth and capable of having liquid water. Using the ESO 3.6-m telescope, a team of Swiss, French and Portuguese scientists discovered a super-Earth about 5 times the mass of the Earth that orbits a red dwarf, already known to harbour a Neptune-mass planet. The astronomers have also strong evidence for the presence of a third planet with a mass about 8 Earth masses.

Artist's impression of the system of three planets surrounding the red dwarf Gliese 581. One of them is the first rocky planet lying in the habitable zone to have been discovered. (Credit: ESO)

This exoplanet - as astronomers call planets around a star other than the Sun -- is the smallest ever found up to now [1] and it completes a full orbit in 13 days. It is 14 times closer to its star than the Earth is from the Sun. However, given that its host star, the red dwarf Gliese 581 [2], is smaller and colder than the Sun -- and thus less luminous -- the planet nevertheless lies in the habitable zone, the region around a star where water could be liquid!

"We have estimated that the mean temperature of this super-Earth lies between 0 and 40 degrees Celsius, and water would thus be liquid," explains Stéphane Udry, from the Geneva Observatory (Switzerland) and lead-author of the paper reporting the result. "Moreover, its radius should be only 1.5 times the Earth's radius, and models predict that the planet should be either rocky -- like our Earth -- or covered with oceans," he adds.

"Liquid water is critical to life as we know it," avows Xavier Delfosse, a member of the team from Grenoble University (France). "Because of its temperature and relative proximity, this planet will most probably be a very important target of the future space missions dedicated to the search for extra-terrestrial life. On the treasure map of the Universe, one would be tempted to mark this planet with an X."

The host star, Gliese 581, is among the 100 closest stars to us, located only 20.5 light-years away in the constellation Libra ("the Scales"). It has a mass of only one third the mass of the Sun. Such red dwarfs are intrinsically at least 50 times fainter than the Sun and are the most common stars in our Galaxy: among the 100 closest stars to the Sun, 80 belong to this class.

"Red dwarfs are ideal targets for the search for low-mass planets where water could be liquid. Because such dwarfs emit less light, the habitable zone is much closer to them than it is around the Sun," emphasizes Xavier Bonfils, a co-worker from Lisbon University. Planets lying in this zone are then more easily detected with the radial-velocity method [3], the most successful in detecting exoplanets.

Two years ago, the same team of astronomers already found a planet around Gliese 581 (see ESO 30/05). With a mass of 15 Earth-masses, i.e. similar to that of Neptune, it orbits its host star in 5.4 days. At the time, the astronomers had already seen hints of another planet. They therefore obtained a new set of measurements and found the new super-Earth, but also clear indications for another one, an 8 Earth-mass planet completing an orbit in 84 days. The planetary system surrounding Gliese 581 contains thus no fewer than 3 planets of 15 Earth masses or less, and as such is a quite remarkable system.

The discovery was made thanks to HARPS (High Accuracy Radial Velocity for Planetary Searcher), perhaps the most precise spectrograph in the world. Located on the ESO 3.6-m telescope at La Silla, Chile, HARPS is able to measure velocities with a precision better than one metre per second (or 3.6 km/h)! HARPS is one of the most successful instruments for detecting exoplanets and holds already several recent records, including the discovery of another 'Trio of Neptunes' (ESO 18/06, see also ESO 22/04).

The detected velocity variations are between 2 and 3 metres per second, corresponding to about 9 km/h! That's the speed of a person walking briskly. Such tiny signals could not have been distinguished from 'simple noise' by most of today's available spectrographs.

"HARPS is a unique planet hunting machine," says Michel Mayor, from Geneva Observatory, and HARPS Principal Investigator. "Given the incredible precision of HARPS, we have focused our effort on low-mass planets. And we can say without doubt that HARPS has been very successful: out of the 13 known planets with a mass below 20 Earth masses, 11 were discovered with HARPS!"

HARPS is also very efficient in finding planetary systems, where tiny signals have to be uncovered. The two systems known to have three low mass planets -- HD 69830 and Gl 581 -- were discovered by HARPS.

"And we are confident that, given the results obtained so far, finding a planet with the mass of the Earth around a red dwarf is within reach," affirms Mayor.

This research is reported in a paper submitted as a Letter to the Editor of Astronomy and Astrophysics ("The HARPS search for southern extra-solar planets : XI. An habitable super-Earth (5 MEarth) in a 3-planet system", by S. Udry et al.)

The team is composed of Stéphane Udry, Michel Mayor, Christophe Lovis, Francesco Pepe, and Didier Queloz (Geneva Observatory, Switzerland), Xavier Bonfils (Lisbonne Observatory, Portugal), Xavier Delfosse, Thierry Forveille, and C.Perrier (LAOG, Grenoble, France), François Bouchy (Institut d'Astrophysique de Paris, France), and Jean-Luc Bertaux (Service d'Aéronomie du CNRS, France)

Notes

[1] Using the radial velocity method, astronomers can only obtain a minimum mass (as it is multiplied by the sine of the inclination of the orbital plane to the line of sight, which is unknown). From a statistical point of view, this is however often close to the real mass of the system. Two other systems have a mass close to this. The icy planet around OGLE-05-390L, discovered by microlensing with a network of telescopes including one at La Silla (ESO 03/06), has a (real) mass of 5.7 Earth masses. It, however, orbits much farther from its small host star than the present one and is hence much colder. The other is one of the planets surrounding the star Gliese 876. It has a minimum mass of 5.89 Earth masses (and a probable real mass of 7.53 Earth masses) and completes an orbit in less than 2 days, making it too hot for liquid water to be present.

[2] Gl 581, or Gliese 581, is the 581th entry in the Gliese Catalogue, which lists all known stars within 25 parsecs (81.5 light years) of the Sun. It was originally compiled by Gliese and published in 1969, and later updated by Gliese and Jahreiss in 1991.

[3] This fundamental observational method is based on the detection of variations in the velocity of the central star, due to the changing direction of the gravitational pull from an (unseen) exoplanet as it orbits the star. The evaluation of the measured velocity variations allows deducing the planet's orbit, in particular the period and the distance from the star, as well as a minimum mass.

Adapted from materials provided by European Southern Observatory, via EurekAlert!, a service of AAAS.

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