Category: Extrasolar Planets

Artist’s conception of the 25-million-year-old protoplanetary disk. Credit: David A, Aguilar (CfA) Click to enlarge
Every rule has an exception. One rule in astronomy, supported by considerable evidence, states that dust disks around newborn stars disappear in a few million years. Most likely, they vanish because the material has collected into full-sized planets. Astronomers have discovered the first exception to this rule – a 25-million-year-old dust disk that shows no evidence of planet formation.

“Finding this disk is as unexpected as locating a 200-year-old person,” said astronomer Lee Hartmann of the Harvard-Smithsonian Center for Astrophysics (CfA), lead author on the paper announcing the find.

The discovery raises the puzzling question of why this disk has not formed planets despite its advanced age. Most protoplanetary disks last only a few million years, while the oldest previously known disks have ages of about 10 million years.

“We don’t know why this disk has lasted so long, because we don’t know what makes the planetary formation process start,” said co-author Nuria Calvet of CfA.

The disk in question orbits a pair of red dwarf stars in the Stephenson 34 system, located approximately 350 light-years away in the constellation Taurus. Data from NASA’s Spitzer Space Telescope shows that its inner edge is located about 65 million miles from the binary stars. The disk extends to a distance of at least 650 million miles. Additional material may orbit farther out where temperatures are too low for Spitzer to detect it.

Astronomers estimate the newfound disk to be about 25 million years old. They calculated the age by modeling the central stars within the system, since stars and disk share the same age. The appearance of the disk itself also supports an advanced age.

“The disk looks a lot different than most other disks we’ve seen. This disk looks a lot more evolved than those around younger stars,” said Hartmann.

Hartmann and Calvet hold opposite opinions about the eventual fate of the disk around Stephenson 34.

“Most stars, by the age of 10 million years, have done whatever they’re going to do,” said Hartmann. “If it hasn’t made planets by now, it probably never will.”

Calvet disagreed. “This disk still has a lot of gas in it, so it may still form giant planets.”

Both astronomers emphasize that such debates are a natural part of the scientific process.

“Some people expect scientists to have all the answers. But research is all about exploring the edge of what is known,” said Hartmann. “That’s what makes it so exciting!”

In the future, Hartmann and Calvet plan to search for more old disks in order to learn why some disks survive so much longer than most others.

“It’s important to find more objects like this because they give us clues about the conditions that influence the formation of planets,” said Calvet.

This research will be published in The Astrophysical Journal Letters.

Original Source: CfA News Release

Artist’s animation shows the view from a hypothetical moon in orbit around the planet. Image credit: NASA. Click to enlarge
A NASA-funded astronomer has discovered a world where the sun sets over the horizon, followed by a second sun and then a third. The new planet, called HD 188753 Ab, is the first known to reside in a classic triple-star system.

“The sky view from this planet would be spectacular, with an occasional triple sunset,” said Dr. Maciej Konacki (MATCH-ee Konn-ATZ-kee) of the California Institute of Technology, Pasadena, Calif., who found the planet using the Keck I telescope atop Mauna Kea mountain in Hawaii. “Before now, we had no clues about whether planets could form in such gravitationally complex systems.”

The finding, reported in this week’s issue of Nature, suggests that planets are more robust than previously believed.

“This is good news for planets,” said Dr. Shri Kulkarni, who oversees Konacki’s research at Caltech. “Planets may live in all sorts of interesting neighborhoods that, until now, have gone largely unexplored.” Kulkarni is the interdisciplinary scientist for NASA’s planned SIM PlanetQuest mission, which will search for signs of Earth-like worlds.

Systems with multiple stars are widespread throughout the universe, accounting for more than half of all stars. Our Sun’s closest star, Alpha Centauri, is a member of a trio.

“Multiple-star systems have not been popular planet-hunting grounds,” said Konacki. “They are difficult to observe and were believed to be inhospitable to planets.”

The new planet belongs to a common class of extrasolar planets called “hot Jupiters,” which are gas giants that zip closely around their parent stars. In this case, the planet whips every 3.3 days around a star that is circled every 25.7 years by a pirouetting pair of stars locked in a 156-day orbit.

The circus-like trio of stars is a cramped bunch, fitting into the same amount of space as the distance between Saturn and our Sun. Such tight living quarters throw theories of hot Jupiter formation into question. Astronomers had thought that hot Jupiters formed far away from their parent stars, before migrating inward.

“In this close-knit system, there would be no room at the outskirts of the parent star system for a planet to grow,” said Konacki.

Previously, astronomers had identified planets around about 20 binary stars and one set of triple stars. But the stars in those systems had a lot of space between them. Most multiple-star arrangements are crowded together and difficult to study.

Konacki overcame this challenge using a modified version of the radial velocity, or “wobble,” planet-hunting technique. In the traditional wobble method, a planet’s presence is inferred by the gravitational tug, or wobble, it induces in its parent star. The strategy works well for single stars or far-apart binary and triple stars, but could not be applied to close-star systems because the stars’ light blends together.

By developing detailed models of close-star systems, Konacki was able to tease apart the tangled starlight. This allowed him to pinpoint, for the first time, the tug of a planet on a star snuggled next to other stars. Of 20 systems examined so far, HD 188753, located 149 light-years away, was the only one found to harbor a planet.

Hot Jupiters are believed to form out of thick disks, or “doughnuts,” of material that swirl around the outer fringes of young stars. The disk material clumps together to form a solid core, then pulls gas onto it. Eventually, the gas giant drifts inward. The discovery of a world under three suns contradicts this scenario. HD 188753 would have sported a truncated disk in its youth, due to the disruptive presence of its stellar companions. That leaves no room for HD 188753’s planet to form, and raises a host of new questions.

The masses of the three stars in HD 188753 system range from two-thirds to about the same mass as our Sun. The planet is slightly more massive than Jupiter.

For artist’s concepts and other graphics, visit http://planetquest.jpl.nasa.gov/ . For information about NASA and agency programs on the Web, visit http://www.nasa.gov/home/index.html .

Original Source: NASA News Release

An extrasolar planet with hypothetical (possible but unproven) water-bearing moons. Image credit: NASA/IPAC/R. Hurt. Click to enlarge
Over the past decade, astronomers using a planet-hunting technique that measures small changes in a star’s speed relative to Earth, have discovered more than 130 extrasolar planets. The first such planets were gas giants, the mass of Jupiter or larger. After several years, the scientists began to detect Saturn-mass planets. And last August, they announced the discovery of a handful of Neptune-mass planets. Could these be super-Earths?

In a recent talk at a symposium on extrasolar planets, Carnegie Institution of Washington astronomer Alan Boss explained the possibilities.

Radial-velocity planet-hunting techniques recently have pushed our discovery capability below the Saturn-mass limit down into what we would call the ice-giant limit.

So we are now able to find planets, close to their host stars, with masses comparable to that of Uranus and Neptune (14 to 17 times the mass of Earth).

In large part this is due to Michel Mayor and his colleagues having a new spectrometer in La Silla, which has unprecedented spectral resolution down to about 1 meter per second or so. And I think Geoff Marcy and Paul Butler’s group are quite close behind that as well.

The interesting question, though, is: What are these things? Are they ice giants that formed several AUs out and migrated in, or are they something else? Unfortunately, we don’t know exactly what their masses are. Even more importantly, we don’t really know what their density is. So they could be 15-Earth-mass rocks, or they could be 15-Earth-mass ice giants.

What we really need to do is to have folks go out and discover another 7 or so. We’ve got 3 so far. If we had 10 altogether, then we’ll have enough that 1 of them, at least, should transit its star and then we’ll be able to get some idea of what its density is.

I think, though, that there’s a good chance that these might actually be a new class of planet altogether: super-Earths. The reason I would argue that is that, at least in 2 of the systems where they’ve been found, these “hot Neptunes” are accompanied by a larger Jupiter-mass planet with a longer-period orbit.

If the lower-mass planets are ice giants that formed far from their stars, unless you have some highly contrived scenario, you wouldn’t imagine them to end up migrating inward, past the larger guys. These systems look more like our own solar system, where you have the low-mass fellows inside of the gas giants.

The planets in a system like our system presumably did not undergo very much migration. So I would claim that perhaps these guys are objects which formed inside the gas giants and only migrated in a little bit, ending up where we can detect them with the short-period spectroscopy surveys.

In support of this idea, there’s some theoretical work from Carnegie’s George Wetherill from almost 10 years ago, now, where he had done some calculations of the accumulation process of rocky planets. He often found there was quite a spread in the masses of what you got out, because accumulation’s a very stochastic process. For the typical parameters he used, at the end of 100 million years or so, he would not only get objects of 1 Earth mass, but also objects ranging up to 3 Earth masses.

Well, at the time, he assumed for his calculations a fairly low surface density at 1 AU, where these planets were forming. Given what we know now, if you want to be able to make a Jupiter at 5 AU using the core-accretion model of planetary formation, you have to crank up the density in the protoplanetary disk by a factor of 7 or so over what Wetherill assumed.

That scales directly with the mass of the planets you’d expect to find as a result. So if you did these calculations over again, assuming this higher initial density, the upper limit on the mass of the inner planets would go from 3 Earth masses, which is what Wetherill got, up to say 21 Earth masses. That is in the range of what we are estimating for these newly discovered hot Neptune-mass objects.

So perhaps what we really are seeing is a new class of objects, super-Earths, rather than ice giants.

Original Source: NASA Astrobiology

Perhaps 1 in 4 stars have planets. Image credit: Hubble. Click to enlarge.
In the past decade, more than 130 extrasolar planets have been discovered to date. Most of these have been found using a technique that measures tiny changes in a star’s radial velocity, the speed of its motion relative to Earth. In a talk at a recent symposium on extrasolar planets, astronomer Alan Boss, of the Carnegie Institution of Washington, presented this overview of the difficult measurements – and the profound discoveries – made by planet-hunters using the radial-velocity technique.

In 1991, Michel Mayor and Antoine Duquennoy published a classic survey of binary stars in our solar neighborhood. They found all the binary companions that they could, but there were another 200 or so G-type stars that didn’t seem to have any binary companions. Subsequently, Michel Mayor, along with Didier Queloz, decided to look at these 200-odd stars, potential solar analogs, to see if they had planetary systems. The technique they used involved looking for stellar wobbles, cyclical changes in the stars’ radial velocity, induced by the gravitational tug of orbiting planets.

In the spring of 1994, they installed a new spectrometer on their telescope at the Haute Provence Observatory, ELODIE, which had a resolution of about 13 meters per second. This was just about the right level to be able to see the velocity wobble, the Doppler wobble, induced in the Sun by a Jupiter-like planet. By the end of 1994 they had noticed a very interesting wobble in a star called 51 Peg.

Unfortunately, 51 Peg at that point was getting closer and closer to the Sun and couldn’t be observed, so they had to take a 6-month sabbatical, and come back in the summer of 1995 and start looking at 51 Peg again. They had an 8-night observing run at the Haute Provence Observatory, and by the end of that observing run, they were ready to go to Nature and publish.

The curve they produced fit a model of 51 Peg, a solar-type star, being orbited by a planet with roughly a half of a Jupiter mass, on a nice, circular orbit. The only problem was that the object had an orbital period of 4.23 days. It was orbiting in at about 0.05 AU, nowhere near where people had been expecting to find Jupiter-mass planets. So it was a bit of a puzzle. But it was clear early on that this had to be a planet, which perhaps had formed farther out and migrated in. That was the only way to explain how it could exist at that location.

The next step was to see if anyone else could reproduce the result. Because, of course, the critical problem with the planet around Barnard’s star was that no one could confirm it. There were several other planet-hunting efforts underway at the time in 1995, but the folks who got to the telescope first were Paul Butler and Geoff Marcy. They were able to confirm 51 Peg’s planet, with even smaller scatter than the original discovery measurements.

We realized at this point that the field of extrasolar planets had truly been born. In October 1995 a new era was entered, where we actually had convincing, solid proof of the existence of extrasolar planets around normal stars.

Now Geoff and Paul had been working in this field for many years. They had actually started seriously around 1987, and so they had a lot of data ready to analyze. They immediately began to reduce all of their data, looking for short period orbits, took some more measurements, and by January of 1996, they were able to announce a couple more planets. One of them, 47 UMa b, was considerably more reassuring a planet than the one discovered orbiting 51 Peg. It was roughly a 2 or 3 Jupiter-mass object orbiting at a distance of 2 or so AU, more like what we were expecting to find based on the planets in our own solar system. We now know that this is a multiple-planet system, but at the time they fit it with a single Keplerian orbit.

Almost all of the known extrasolar planets have been found using this radial-velocity technique; roughly 117 planets have been discovered that way. But there’s another way of finding planets, transit detection. The first transit detection was achieved by David Charboneau and colleagues and separately by Greg Henry and colleagues in 2000. This was a planet which had been found originally by radial velocity, but then these other researchers went on and did both ground-based and later Hubble photometry of the host star and found a really wonderful light curve, indicative of the planet passing in front of the star, dimming its light slightly. The initial detection by Charbonneau’s team was done, believe it or not, using a 4-inch telescope in a parking lot in Boulder, Colorado.

The dip in the star’s light amplitude is about 1.5 percent, so it’s truly amazing that this very first transit detection could have been made by a good amateur telescope. When HST went back and re-did the photometry with much higher precision, it produced an incredibly beautiful light curve, which is so precise you could use it to try to search for moons around the planet and place limits on how large they could be.

So transits are now coming into their own. I think they’re the second leading way of finding planets. Six planets have been discovered by transits now.

Original source: NASA Astrobiology

Artist illustration of the planet orbiting the sun-like star HD 149026. Image credit: U.C. Santa Cruz. Click to enlarge.
NASA researchers recently discovered the largest solid core ever found in an extrasolar planet, and their discovery confirms a planet formation theory.

“For theorists, the discovery of a planet with such a large core is as important as the discovery of the first extrasolar planet around the star 51 Pegasi in 1995,” said Shigeru Ida, theorist from the Tokyo Institute of Technology, Japan.

When a consortium of American, Japanese and Chilean astronomers first looked at this planet, they expected one similar to Jupiter. “None of our models predicted that nature could make a planet like the one we are studying,” said Bun’ei Sato, consortium member and postdoctoral fellow at Okayama Astrophysical Observatory, Japan.

Scientists have rarely had opportunities like this to collect such solid evidence about planet formation. More than 150 extrasolar planets have been discovered by observing changes in the speed of a star, as it moves toward and away from Earth. The changes in speed are caused by the gravitational pull of planets.

This planet also passes in front of its star and dims the starlight. “When that happens, we are able to calculate the physical size of the planet, whether it has a solid core, and even what its atmosphere is like,” said Debra Fischer. She is consortium team leader and professor of astronomy at San Francisco State University, Calif.

The planet, orbiting the sun-like star HD 149026, is roughly equal in mass to Saturn, but it is significantly smaller in diameter. It takes just 2.87 days to circle its star, and the upper atmosphere temperature is approximately 2,000 degrees Fahrenheit. Modeling of the planet’s structure shows it has a solid core approximately 70 times Earth’s mass.

This is the first observational evidence that proves the “core accretion” theory about how planets are formed. Scientists have two competing but viable theories about planet formation.

In the “gravitational instability” theory, planets form during a rapid collapse of a dense cloud. With the “core accretion” theory, planets start as small rock-ice cores that grow as they gravitationally acquire additional mass. Scientists believe the large, rocky core of this planet could not have formed by cloud collapse. They think it must have grown a core first, and then acquired gas.

“This is a confirmation of the core accretion theory for planet formation and evidence that planets of this kind should exist in abundance,” said Greg Henry, an astronomer at Tennessee State University, Nashville. He detected the dimming of the star by the planet with his robotic telescopes at Fairborn Observatory in Mount Hopkins, Arizona.

Original Source: NASA News Release

Hubble image of the ring around Fomalhaut. Image credit: Hubble. Click to enlarge.
NASA Hubble Space Telescope’s most detailed visible-light image ever taken of a narrow, dusty ring around the nearby star Fomalhaut (HD 216956), offers the strongest evidence yet that an unruly and unseen planet may be gravitationally tugging on the ring.

Hubble unequivocally shows that the center of the ring is a whopping 1.4 billion miles (15 astronomical units) away from the star. This is a distance equal to nearly halfway across our solar system. The most plausible explanation, astronomers said, is that an unseen planet moving in an elliptical orbit is reshaping the ring with its gravitational pull. The geometrically striking ring, tilted obliquely toward Earth, would not have such a great offset if it were simply being influenced by Fomalhaut’s gravity alone.

An offset of the ring center from the star has been inferred from previous and longer wavelength observations using submillimeter telescopes on Mauna Kea, Hawaii, the Spitzer Space Telescope, Caltech’s Submillimeter Observatory and applying theoretical modeling and physical assumptions. Now Hubble’s sharp images directly reveal the ring’s offset from Fomalhaut.

These new observations provide strong evidence that at least one unseen planetary mass object is orbiting the star. Hubble would have detected an object larger than a planet, such as a brown dwarf. “Our new Hubble images confirm those earlier hypotheses that proposed a planet was perturbing the ring,” said Paul Kalas of the University of California at Berkeley. The ring is similar to our solar system’s Kuiper Belt, a vast reservoir of icy material left over from the formation of our solar system planets.

The observations offer insights into our solar system’s formative years, when the planets played a game of demolition derby with the debris left over from the formation of our planets, gravitationally scattering many objects across space. Some icy material may have collided with the inner solar system planets, irrigating them with water formed in the colder outer solar system. Other debris may have traveled outward, forming the Kuiper Belt and the Oort Cloud, a spherical cloud of material surrounding the solar system.

Only Hubble has the exquisite optical resolution to resolve that the ring’s inner edge is sharper than its outer edge, a telltale sign that an object is gravitationally sweeping out material like a plow clearing away snow. Another classic signature of a planet’s influence is the ring’s relatively narrow width, about 2.3 billion miles (25 astronomical units). Without an object to gravitationally keep the ring material intact, astronomers said, the particles would spread out much wider.

“What we see in this ring is similar to what is seen in the Cassini spacecraft images of Saturn’s narrow rings. In those images, Saturn’s moons are ‘shepherding’ the ring material and keeping the ring from spreading out,” Kalas said.

The suspected planet may be orbiting far away from Fomalhaut, inside the dust ring’s inner edge, between 4.7 billion and 6.5 billion miles (50 to 70 astronomical units) from the star. The ring is 12 billion miles (133 astronomical units) from Fomalhaut, which is much farther away than our outermost planet Pluto is from the Sun. These Hubble observations do not detect the putative planet directly, so the astronomers cannot measure its mass. They will, instead, conduct computer simulations of the ring’s dynamics to estimate the planet’s mass.

Kalas and collaborators James R. Graham of the University of California at Berkeley and Mark Clampin of the NASA Goddard Space Flight Center in Greenbelt, Md., will publish their findings in the June 23, 2005 issue of the journal Nature.

Fomalhaut, a 200-million-year-old star, is a mere infant compared to our own 4.5-billion-year-old Sun. It resides 25 light-years away from the Sun. Located in the constellation Piscis Austrinus (the Southern Fish), the Fomalhaut ring is ten times as old as debris disks seen previously around the stars AU Microscopii and Beta Pictoris, where planets may still be forming. If our solar system is any example, planets should have formed around Fomalhaut within tens of millions of years after the birth of the star.

The Hubble images also provide a glimpse of the outer planetary region surrounding a star other than our Sun. Many of the more than 100 planets detected beyond our solar system are orbiting close to their stars. Most of the current planet-detecting techniques favor finding planets that are close to their stars.

“The size of Fomalhaut’s dust ring suggests that not all planetary systems form and evolve in the same way ? planetary architectures can be quite different from star to star,” Kalas explained. “While Fomalhaut’s ring is analogous to the Kuiper Belt, its diameter is four times greater than that of the Kuiper Belt.”

The astronomers used the Advanced Camera for Surveys’ (ACS) coronagraph aboard Hubble to block out the light from the bright star so they could see details in the faint ring.

“The ACS’s coronagraph offers high contrast, allowing us to see the ring’s structure against the extremely bright glare from Fomalhaut,” Clampin said. “This observation is currently impossible to do at visible wavelengths without the Hubble Space Telescope. The fact that we were able to detect it with Hubble was unexpected, but impressive.”

Kalas and his collaborators used Hubble over a five-month period in 2004 ? May 17, Aug. 2, and Oct. 27 ? to map the ring’s structure. One side of the ring has yet to be imaged because it extended beyond the ACS camera’s field of view. The astronomers will use Hubble again this summer to map the entire ring. They expect that the additional Hubble data will reveal whether or not the ring has any gaps, which could have been carved out by the gravitational influence of an unseen body. The longer, deeper exposures also may show whether the ring has an even wider diameter than currently seen. In addition, the astronomers will measure the ring’s colors to determine its physical properties, including its composition.

Previous thermal emission maps of Fomalhaut showed that one side of the ring is warmer than the other side, implying that the ring is off center by about half the distance measured by Hubble. This difference might be explained by the fact that Hubble’s ACS images of the ring’s structure are 100 times sharper than the longer wavelength observations, and hence, yield a much more accurate result. Or the discrepancy might imply that the ring’s size looks different at other wavelengths.

Fomalhaut’s dust ring was discovered in 1983 in observations made by NASA’s Infrared Astronomical Satellite (IRAS). The system is a compelling target for future telescopes such as the James Webb Space Telescope and the Terrestrial Planet Finder, Kalas said.

Original Source: Hubble News Release

Artist illustration of the rocky planet around the M dwarf Gliese 876. Image credit: NSF. Click to enlarge.
In the land rush known as extrasolar planet hunting, the most prized real estate is advertised as “Earth-like.” On Monday, June 13, scientists raced to plant their flag on a burning hunk of rock orbiting a red star.

This newly discovered planet is about seven times the mass of Earth, and therefore the smallest extrasolar planet found to orbit a main sequence, or “dwarf” star (stars, like our sun, that burn hydrogen).

There are even smaller planets known to exist beyond our solar system, but they have the misfortune to encircle pulsars, those rapidly spinning husks of dying stars. Such planets aren’t thought to be remotely habitable, due to the intense radiation emitted by pulsars.

Planets that are ten Earth masses or less are thought to be rocky, while more massive planets are probably gaseous, since their stronger gravity means they collect and retain more gas during planetary formation. 155 extrasolar planets have been found so far, but most of them have masses that are more comparable to gaseous Jupiter than rocky Earth (Jupiter is 318 times the mass of Earth).

Although this new planet is advertised as Earth-like because of its relatively low mass, earthlings wouldn’t want to rent a house there any time soon. For one thing, the house would melt. The surface temperatures estimated for this planet – 200 to 400 degrees Celsius (400 to 750 degrees Fahrenheit) – are due to the planet’s kissing-close distance from its star.

The planet resides a mere 0.021 AU from the star Gliese 876 (1 AU is the distance between the Earth and the sun), and completes an orbit in less then two Earth days. The closest planet to the sun in our own solar system – blazing hot Mercury – is nearly 20 times further away, orbiting at about 0.4 AU.

“Because the planet is in a two-day orbit, it is heated to oven-like temperatures, so we do not expect life,” says science team member Paul Butler of the Carnegie Institution of Washington.

In our solar system, the habitable zone – the temperate region where water could exist as a liquid on a planet’s surface – is roughly 0.95 to 1.37 AU, or between the orbits of Venus and Mars. The star Gliese 876 is about 600 times less luminous than our sun, so the proposed habitable zone is much closer in, roughly between 0.06 and 0.22 AU.

At 0.021 AU, the new planet is too close to the star to be in the habitable zone, and it also is subjected to greater amounts of high energy radiation like ultraviolet light and X-rays. While red dwarfs like Gliese 876 emit lower levels of UV than stars like our sun, they do emit violent X-ray flares.

Another complication from such a close orbit is that the planet may be tidally locked, with the same side of the planet always facing the star. Unless there is a substantial atmosphere to distribute heat, one side of the planet will be overcooked while the other will remain cold.

Gliese 876 is thought to be about 11 billion years old, making it more than twice as old as our sun. But in a way, Gliese is a teenager to our sun’s middle-aged adult. G-class stars like our sun live about 10 billion years, while M-class red dwarfs are thought to live for 100 billion years (older than the age of the universe!).

Science team member Geoff Marcy of the University of California, Berkeley, says that M stars take a long time to cool off and shrink down to their main sequence size and luminosity. He says that if the planet migrated inwards to its present day close orbit, it probably made this move during the first few million years, and then was subjected to much more radiation than at present for hundreds of millions of years.

Gliese 876 is thought to be metal-poor (to an astronomer, any element heavier than hydrogen and helium is classified as a “metal”). The formation of planets may be related to the metallicity of the star, since both the star and the planets form from the same original material. So a rocky planet like the Earth, made out of elements such as silicates and iron, is expected to orbit a star that is metal-rich.

Despite being metal-poor, Gliese 876 is a multiple planet system. Two gas giant planets are known to orbit Gliese 876: the outermost planet is nearly twice the mass of Jupiter, and orbits at 0.21 AU; the middle planet is about half the mass of Jupiter, orbiting at 0.13 AU.

“The whole planetary system is sort of a miniature of our solar system,” says Marcy. “The star is small, the orbits are small, and in closer is the smallest of them, just as the architecture is in our own solar system, with the smallest planets orbiting inward of the giants.”

We have a lot more elbow room in our solar system. Mercury is further away from the sun than the distances of all these planets combined. The planets in the Gliese 876 system are so close together, they gravitationally interact with each other. This sort of gravitational tug of war was how the scientists were able to detect the planets in the first place.

Over the course of an orbit, planets will gravitationally pull on their star from different sides. Scientists measure the resulting shift in star light to determine the existence of orbiting planets.

To learn more about Gliese 876’s smallest planet, scientists would need to use another planet-hunting technique called transit photometry. This method looks at how a star’s light seems to dip when a planet passes in front of the star from our field of view. The eclipse of the orbiting planet allows astronomers to determine that planet’s mass and radius. Pinning down those numbers indicates the planet’s density, which then suggests what the planet is made of, and whether the planet is rocky or gaseous.

Transit photometry can’t be used to tell us anything about planets orbiting Gliese 876, however, because the system is inclined 50 degrees from our point of view. This angle means the planets won’t block any of the starlight that reaches Earth.

Red dwarfs are the most common type of star in our galaxy, comprising about 70 percent of all stars. Yet out of the 150 red dwarfs they have studied over the years, Marcy and Butler only have found planets orbiting two of them. Because most of the planets found so far are gas giants, this could mean that red dwarfs are less apt to harbor those kinds of worlds.

Marcy says they will continue to monitor Gliese 876 for any hints of a fourth or fifth planet. “This will definitely be one of our favorite stars from now on.”

A Race to the Finish Line
The research paper describing this discovery has been submitted to the Astrophysical Journal. The scientists say they received a favorable preliminary referee’s report, and they expect their paper will be accepted and then published in a few months. During Monday’s press conference, the scientists were asked why they decided to publicize their finding now, before the paper had been accepted for publication. Was it done to beat out other planet hunters who might be hot on their heels?

Marcy replied that they wanted to prevent news of their discovery from leaking out. “We knew about it three years ago, we’ve been following it quietly, carefully, guarding the secret while we double and triple checked. Then about a month ago I talked with Michael Turner here, people at NSF (National Science Foundation), and jointly we decided that this discovery was so extraordinary, maybe what you would call a milestone in planetary science, that it was difficult to imagine keeping the lid on this for very much longer. So we decided that rather than have it leak out to the news media, and be dribbled around, with one newspaper learning about it early and so on, that it would be better to quickly announce this.”

Marcy then launched into a defense for why he believed their finding is correct, and he was quickly backed by his fellow team members. However, the accuracy of their finding had not been questioned. Perhaps their early announcement, combined with the need for secrecy beforehand, is evidence of the intense competition that has marked planet hunting since the beginning.

The first extrasolar planet discovery was announced October 5, 1995 by Michel Mayor and Didier Queloz of the Geneva Observatory, and Marcy and Butler confirmed the observations the following week. A recent example of the competition to grab other extrasolar planet “firsts” occurred last summer, when on August 25, 2004, Mayor, Nuno Santos, and colleagues announced the discovery of the first extrasolar Neptune-mass planet — at the time the smallest extrasolar planet known to orbit a sun-like star. This announcement came less than a week before two other Neptune-mass planet discoveries were announced by Marcy and Butler.

Mayor and his colleagues also have studied Gliese 876. At an astronomy conference in June 1998, Mayor and Marcy each independently announced the detection of the more massive gas giant orbiting this star. Marcy and Butler were first to follow up on this finding, announcing the discovery of the star’s second gas giant planet in 2001.

The Kepler mission, due to launch in June 2008, will search for terrestrial planets orbiting distant stars. The mission defines an Earth-size planet as being between 0.5 and 2.0 Earth masses, or between 0.8 and 1.3 Earth’s diameter. Planets between 2 and 10 Earth masses, such as the planet announced on Monday, are defined as Large Terrestrial planets.

Original Source: NASA Astrobiology

Artist illustration of the rocky planet around the M dwarf Gliese 876. Image credit: NSF. Click to enlarge.
Taking a major step forward in the search for Earth-like planets beyond our own solar system, a team of astronomers has announced the discovery of the smallest extrasolar planet yet detected. About seven-and-a-half times as massive as Earth, with about twice the radius, it may be the first rocky planet ever found orbiting a normal star not much different from our Sun.

All of the nearly 150 other extrasolar planets discovered to date around normal stars have been larger than Uranus, an ice-giant about 15 times the mass of the Earth.

“We keep pushing the limits of what we can detect, and we’re getting closer and closer to finding Earths,” said team member Steven Vogt, a professor of astronomy and astrophysics at the University of California, Santa Cruz.

?Today’s results are an important step toward answering one of the most profound questions that mankind can ask: Are we alone in the universe?? said Michael Turner, head of the Mathematical and Physical Sciences Directorate at the National Science Foundation, which provided partial funding for the research.

The newly-discovered ?super-Earth? orbits the star Gliese 876, located just 15 light years away in the direction of the constellation Aquarius. This star also possesses two larger, Jupiter-size planets. The new planet whips around the star in a mere two days, and is so close to the star’s surface that its temperature probably tops 400 to 750 degrees Fahrenheit (200 to 400 degrees Celsius)?oven-like temperatures far too hot for life as we know it.

Nevertheless, the ability to detect the tiny wobble that the planet induces in the star gives astronomers confidence that they will be able to detect even smaller rocky planets in orbits more hospitable to life.

“This is the smallest extrasolar planet yet detected and the first of a new class of rocky terrestrial planets,” said team member Paul Butler of the Carnegie Institution of Washington. “It’s like Earth’s bigger cousin.”

The team measures a minimum mass for the planet of 5.9 Earth masses, orbiting Gliese 876 with a period of 1.94 days at a distance of 0.021 astronomical units (AU), or 2 million miles.

Though the team has no direct proof that the planet is rocky, its low mass precludes it from retaining gas like Jupiter. Three other purported rocky planets have been reported, but they orbit a pulsar, the flashing corpse of an exploded star.

“This planet answers an ancient question,” said team leader Geoffrey Marcy, professor of astronomy at the University of California, Berkeley. “Over 2,000 years ago, the Greek philosophers Aristotle and Epicurus argued about whether there were other Earth-like planets. Now, for the first time, we have evidence for a rocky planet around a normal star.”

Marcy, Butler, theoretical astronomer Jack Lissauer of NASA/Ames Research Center, and post-doctoral researcher Eugenio J. Rivera of the University of California Observatories/Lick Observatory at UC Santa Cruz presented their findings today (Monday, June 13) during a press conference at NSF in Arlington, Va.

Their research, conducted at the Keck Observatory in Hawaii, was supported by NSF, the National Aeronautics and Space Administration, the University of California and the Carnegie Institution of Washington.

A paper detailing the results has been submitted to The Astrophysical Journal. Coauthors on the paper are Steven Vogt and Gregory Laughlin of the Lick Observatory at the University of California, Santa Cruz; Debra Fischer of San Francisco State University; and Timothy M. Brown of NSF?s National Center for Atmospheric Research in Boulder, Colorado.

Gliese 876 (or GJ 876) is a small, red star known as an M dwarf ? the most common type of star in the galaxy. It is located in the Aquarius constellation, and, at about one-third the mass of the sun, is the smallest star around which planets have been discovered. Butler and Marcy detected the first planet there in 1998; it proved to be a gas giant about twice the mass of Jupiter. Then, in 2001, they reported a second planet, another gas giant about half the mass of Jupiter. The two are in resonant orbits, the outer planet taking 60 days to orbit the star, twice the period of the inner giant planet.

Lissauer and Rivera have been analyzing Keck data on the Gliese 876 system in order to model the unusual motions of the two known planets, and three years ago got an inkling that there might be a smaller, third planet orbiting the star. In fact, if they hadn’t taken account of the resonant interaction between the two known planets, they never would have seen the third planet.

“We had a model for the two planets interacting with one another, but when we looked at the difference between the two-planet model and the actual data, we found a signature that could be interpreted as a third planet,” Lissauer said.

A three-planet model consistently gave a better fit to the data, added Rivera. “But because the signal from this third planet was not very strong, we were very cautious about announcing a new planet until we had more data,” he said.

Recent improvements to the Keck Telescope’s high-resolution spectrometer (HIRES) provided crucial new data. Vogt, who designed and built HIRES, worked with the technical staff in the UC Observatories/Lick Observatory Laboratories at UC Santa Cruz to upgrade the spectrometer’s CCD (charge coupled device) detectors last August.

“It is the higher precision data from the upgraded HIRES that gives us confidence in this result,” Butler said.

The team now has convincing data for the planet orbiting very close to the star, at a distance of about 10 stellar radii. That’s less than one-tenth the size of Mercury’s orbit in our solar system.

“In a two-day orbit , it’s about 200 degrees Celsius too hot for liquid water,” Butler said. “That tends to lead us to the conclusion that the most probable composition of this thing is like the inner planets of this solar system ? a nickel-iron rock, a rocky planet, a terrestrial planet.”

“The planet’s mass could easily hold onto an atmosphere,” noted Laughlin, an assistant professor of astronomy at UC Santa Cruz. “It would still be considered a rocky planet, probably with an iron core and a silicon mantle. It could even have a dense steamy water layer. I think what we are seeing here is something that’s intermediate between a true terrestrial planet like the Earth and a hot version of the ice giants Uranus and Neptune.”

Combined with improved computer software, the new CCD (charge coupled device) detectors designed by this team for Keck’s HIRES spectrometer can now measure the Doppler velocity of a star to within one meter per second ? human walking speed ? instead of the previous precision of three meters per second. This improved sensitivity will allow the planet-hunting team to detect the gravitational effect of an Earth-like planet within the habitable zone of M dwarf stars like Gliese 876.

“We are pushing a whole new regime at Keck to achieve one meter per second precision, triple our old precision, that should also allow us to see Earth-mass planets around sun-like stars within the next few years,” Butler said.

“Our UC Santa Cruz and Lick Observatory team has done an enormous amount of optical and technical and detector work to make the Keck telescope a rocky planet hunter, the best one in the world,” Marcy added.

Lissauer also is excited by another feat reported in the paper submitted to the journal. For the first time, he, Rivera and Laughlin have determined the line-of-sight inclination of the orbit of the stellar system solely from the observed Doppler wobble of the star. Using dynamical models of how the two Jupiter-size planets interact, they were able to calculate the masses of the two giant planets from the observed shapes and precession rates of their oval orbits. Precession is the slow turning of the long axis of a planet’s elliptical orbit.

They showed that the orbital plane is tilted 40 degrees to our line of sight. This allowed the team to estimate the most likely mass of the third planet as seven and a half Earth masses.

“There’s more dynamical modeling involved in this study than any previous study, much more,” Lissauer said.

The team plans to continue to observe the star Gliese 876, but is eager to find other terrestrial planets among the 150 or more M dwarf planets they observe regularly with Keck.

“So far we find almost no Jupiter-mass planets among the M dwarf stars we’ve been observing, which suggests that, instead, there is going to be a large population of smaller mass planets,” Butler noted.

Original Source: Carnegie Institute News Release

Artist interpretation of an extrasolar planet. Image credit: NASA. Click to enlarge.
An international collaboration featuring Ohio State University astronomers has detected a planet in a solar system that, at roughly 15,000 light years from Earth, is one of the most distant ever discovered.
Andrew Gould

In a time when technology is starting to make such finds almost commonplace, this new planet — which is roughly three times the size of Jupiter — is special for several reasons, said Andrew Gould, professor of astronomy at Ohio State .

The technique that astronomers used to find the planet worked so well that he thinks it could be used to find much smaller planets — Earth-sized planets, even very distant ones.

And because two amateur astronomers in New Zealand helped detect the planet using only their backyard telescopes, the find suggests that anyone can become a planet hunter.

Gould and his colleagues have submitted a paper announcing the planet to Astrophysical Journal Letters, and have posted the paper on a publicly available Internet preprint server, http://arXiv.org . The team has secured use of NASA’s Hubble Space Telescope in late May to examine the star that the planet is orbiting.

The astronomers used a technique called gravitational microlensing, which occurs when a massive object in space, like a star or even a black hole, crosses in front of a star shining in the background. The object’s strong gravitational pull bends the light rays from the more distant star and magnifies them like a lens. Here on Earth, we see the star get brighter as the lens crosses in front of it, and then fade as the lens gets farther away.
Because the scientists were able to monitor the light signal with near-perfect precision, Gould thinks the technique could easily have revealed an even smaller planet. “If an Earth-mass planet was in the same position, we would have been able to detect it,” he said.

On March 17, 2005, Andrzej Udalski, professor of astronomy at Warsaw University and leader of the Optical Gravitational Lensing Experiment, or OGLE, noticed that a star located thousands of light years from Earth was starting to move in front of another star that was even farther away, near the center of our galaxy. A month later, when the more distant star had brightened a hundred-fold, astronomers from OGLE and from Gould’s collaboration (the Microlensing Follow Up Network, or MicroFUN) detected a new pattern in the signal — a rapid distortion of the brightening — that could only mean one thing.

“There’s absolutely no doubt that the star in front has a planet, which caused the deviation we saw,” Gould said.

Because the scientists were able to monitor the light signal with near-perfect precision, Gould thinks the technique could easily have revealed an even smaller planet.

“If an Earth-mass planet was in the same position, we would have been able to detect it,” he said.

OGLE finds more than 600 microlensing events per year using a dedicated 1.3-meter telescope at Las Campanas Observatory in Chile (operated by Carnegie Institution of Washington). MicroFUN is a collaboration of astronomers from the US, Korea, New Zealand, and Israel that picks out those events that are most likely to reveal planets and monitors them from telescopes around the world.

“That allows us to watch these events 24/7,” Gould said. “When the sun rises at one location, we continue to monitor from the next.”

Two of these telescopes belong to two avid New Zealand amateur astronomers who were recruited by the MicroFUN team. Grant Christie of Auckland used a 14-inch telescope, and Jennie McCormick of Pakuranga used a 10-inch telescope. Both share co-authorship on the paper submitted to Astrophysical Journal Letters.

Two other collaborations — the Probing Lensing Anomalies NETwork (PLANET) and Microlensing Observations in Astrophysics (MOA) — also followed the event and contributed to the journal paper.

Ohio State scientists on the project included Darren DePoy and Richard Pogge, both professors of astronomy, and Subo Dong, a graduate student. Other partners hail from Warsaw University in Poland, Princeton University, Harvard-Smithsonian Center for Astrophysics, Universidad de Concepci?n in Chile, University of Manchester, California Institute of Technology, American Museum of Natural History, Chungbuk National University in Korea, Korea Astronomy and Space Science Institute, Massy University in New Zealand, Nagoya University in Japan, and the University of Auckland in New Zealand.

This is the second planet that astronomers have detected using microlensing. The first one, found a year ago, is estimated to be at a similar distance.

Gould’s initial estimate is that the new planet is approximately 15,000 light years away, but he will need more data to refine that distance, he said. A light year is the distance light travels in a year — approximately six trillion miles.

The OGLE collaboration is funded by the Polish Ministry of Scientific Research and Information Technology, the Foundation for Polish Science, the National Science Foundation, and NASA. Some MicroFUN team members received funding from the National Science Foundation, Harvard College Observatory, the Korea Science and Engineering Foundation, and the Korea Astronomy and Space Science Institute.

Original Source: OSU News Release