Category: Extrasolar Planets

Correlation between the heavy elements in transiting planets and the metallicity of their parents. Image credit: A&A. Click to enlarge
Of the 188 extrasolar planets discovered, 10 are transits; we see them because they dim their parent star as they pass in front. This gives astronomers an opportunity to study the actual composition of these planets. European astronomers have discovered that the metal content of these “hot Jupiters” depends on the amount of metal in their parent star, which changes the size of their cores.

A team of European astronomers, led by T. Guillot (CNRS, Observatoire de la Cote d’Azur, France), will publish a new study of the physics of Pegasids (also known as hot Jupiters) in Astronomy & Astrophysics. They found that the amount of heavy elements in Pegasids is correlated to the metallicity of their parent stars. This is a first step in understanding the physical nature of the extrasolar planets.

Up to now, astronomers have discovered 188 extrasolar planets, among which 10 are known as “transiting planets”. These planets pass between their star and us at each orbit. Given the current technical limitations, the only transiting planets that can be detected are giant planets orbiting close to their parent star known as “hot Jupiters” or Pegasids. The ten transiting planets known thus far have masses between 110 and 430 Earth masses (for comparison, Jupiter, with 318 Earth masses, is the most massive planet in our Solar System).

Although rare, transiting planets are the key to understanding planetary formation because they are the only ones for which both the mass and radius can be determined. In principle, the obtained mean density can constrain their global composition. However, translating a mean density into a global composition needs accurate models of the internal structure and evolution of planets. The situation is made difficult by our relatively poor knowledge of the behaviour of matter at high pressures (the pressure in the interiors of giant planets is more than a million times the atmospheric pressure on Earth). Of the nine transiting planets known up to April 2006, only the least massive one could have its global composition determined satisfactorily. It was shown to possess a massive core of heavy elements, about 70 times the mass of the Earth, with a 40 Earth-mass envelope of hydrogen and helium. Of the remaining eight planets, six were found to be mostly made up of hydrogen and helium, like Jupiter and Saturn, but their core mass could not be determined. The last two were found to be too large to be explained by simple models.

Considering them as an ensemble for the first time, and accounting for the anomalously large planets, Tristan Guillot and his team found that the nine transiting planets have homogeneous properties, with a core mass ranging from 0 (no core, or a small one) up to 100 times the mass of the Earth, and a surrounding envelope of hydrogen and helium. Some of the Pegasids should therefore contain larger amounts of heavy elements than expected. When comparing the mass of heavy elements in the Pegasids to the metallicity of the parent stars, they also found a correlation to exist, with planets born around stars that are as metal-rich as our Sun and that have small cores, while planets orbiting stars that contain two to three times more metals have much larger cores. Their results will be published in Astronomy & Astrophysics.

Planet formation models have failed to predict the large amounts of heavy elements found this way in many planets, so these results imply that they need revising. The correlation between stellar and planetary composition has to be confirmed by further discoveries of transiting planets, but this work is a first step in studying the physical nature of extrasolar planets and their formation. It would explain why transiting planets are so hard to find, to start with. Because most Pegasids have relatively large cores, they are smaller than expected and more difficult to detect in transit in front of their stars. In any case, this is very promising for the CNES space mission COROT to be launched in October, which should discover and lead to characterization of tens of transiting planets, including smaller planets and planets orbiting too far from their star to be detected from the ground.

What of the tenth transiting planet? XO-1b was announced very recently and is also found to be an anomalously large planet orbiting a star of solar metallicity. Models imply that it has a very small core, so that this new discovery strengthens the proposed stellar-planetary metallicity correlation.

Original Source: NASA Astrobiology

Artist’s Concept of Transiting Planet XO-1b. Image credit: NASA/ESA/STScI. Click to enlarge
Amateur astronomers have used inexpensive equipment to discover a Jupiter-sized planet orbiting a Sun-like star 600 light-years away. The team used the “transit method”, to watch how a star dims slightly as a planet passes in front. An automated telescope observed tens of thousands of bright stars, and then the team chose a few dozen promising candidates. The new planet, dubbed X0-1b is the 10th planet ever discovered using the transit method.

An international team of professional and amateur astronomers, using simple off-the-shelf equipment to trawl the skies for planets outside our solar system, has hauled in its first “catch.”

The astronomers discovered a Jupiter-sized planet orbiting a Sun-like star 600 light-years from Earth in the constellation Corona Borealis. The team, led by Peter McCullough of the Space Telescope Science Institute in Baltimore, Md., includes four amateur astronomers from North America and Europe.

Using modest telescopes to search for extrasolar planets allows for a productive collaboration between professional and amateur astronomers that could accelerate the planet quest.

“This discovery suggests that a fleet of modest telescopes and the help of amateur astronomers can search for transiting extrasolar planets many times faster than we are now,” McCullough said. The finding has been accepted for publication in the Astrophysical Journal.

McCullough deployed a relatively inexpensive telescope made from commercial equipment to scan the skies for extrasolar planets. Called the XO telescope, it consists of two 200-millimeter telephoto camera lenses and looks like a pair of binoculars. The telescope is on the summit of the Haleakala volcano, in Hawaii.

“To replicate the XO prototype telescope would cost $60,000,” McCullough explained. “We have spent far more than that on software, in particular on designing and operating the system and extracting this planet from the data.”

McCullough’s team found the planet, dubbed X0-1b, by noticing slight dips in the star’s light output when the planet passed in front of the star, called a transit. The light from the star, called XO-1, dips by approximately 2 percent when the planet XO-1b passes in front of it. The observation also revealed that X0-1b is in a tight four-day orbit around its parent star.

Although astronomers have detected more than 180 extrasolar planets, X0-1b is only the tenth planet discovered using the transit method. It is the second planet found using telephoto lenses. The first, dubbed TrES-1, was reported in 2004. The transit method allows astronomers to determine a planet’s mass and size. Astronomers use this information to deduce the planet’s characteristics, such as its density.

The team confirmed the planet’s existence by using the Harlan J. Smith Telescope and the Hobby-Eberly Telescope at the University of Texas’s McDonald Observatory to measure the slight wobble induced by the planet on its parent star. This so-called radial-velocity method allowed the team to calculate a precise mass for the planet, which is slightly less than that of Jupiter (about 0.9 Jupiter masses). The planet also is much larger than its mass would suggest. “Of the planets that pass in front of their stars, XO-1b is the most similar to Jupiter yet known, and the star XO-1 is the most similar to the Sun,” McCullough said, although he was quick to add, “but XO-1b is much, much closer to its star than Jupiter is to the Sun.”

The astronomer’s innovative technique of using relatively inexpensive telescopes to look for eclipsing planets favors finding planets orbiting close to their parent stars. The planet also must be large enough to produce a measurable dip in starlight.

The planet is the first discovered in McCullough’s three-year search for transiting extrasolar planets. The planet quest is underwritten by a grant from NASA’s Origins program.

McCullough’s planet-finding technique involves nightly sweeps of the sky using the XO telescope in Hawaii to note the brightness of the stars it encounters. A computer software program sifts through many thousands of stars every two months looking for tiny dips in the stars’ light, the signature of a possible planetary transit. The computer comes up with a few hundred possibilities. From those candidates, McCullough and his team select a few dozen promising leads. He passes these stars on to the four amateur astronomers to study the possible transits more carefully.

From September 2003 to September 2005, the XO telescope observed tens of thousands of bright stars. In that time, his team of amateur astronomers studied a few dozen promising candidate stars identified by McCullough and his team. The star X0-1 was pegged as a promising candidate in June 2005. The amateur astronomers observed it in June and July 2005, confirming that a planet-sized object was eclipsing the star. McCullough’s team then turned to the McDonald Observatory in Texas to obtain the object’s mass and verify it as a planet. He received the news of the telescope’s observation at 12:06 a.m. Feb. 16, 2006, from Chris Johns-Krull, a friend and colleague at Rice University.

“It was a wonderful feeling because the team had worked for three years to find this one planet,” McCullough explained. “The discovery represents a few bytes out of nearly a terabyte of data: It’s like trying to distill gold out of seawater.”

The discovery also has special familial significance for the astronomer. “My father’s mentor was Harlan J. Smith, the man whose ambition and hard work produced the telescope that we used to acquire the verifying data.”

McCullough believes the newly found planet is a perfect candidate for study by the Hubble and Spitzer space telescopes. Hubble can measure precisely the star’s distance and the planet’s size. Spitzer can actually see the infrared radiation from the planet. By timing the disappearance of the planet behind the star, Spitzer also can measure the “ellipticity,” or “out-of-roundness,” of the planet’s orbit. If the orbit is elliptical, then the varying gravitational force would result in extra heating of the planet, expanding its atmosphere and perhaps explaining why the object’s diameter seems especially large for a body of its calculated mass.

“By timing the planet’s passages across the star, both amateur and professional astronomers might be lucky enough to detect the presence of another planet in the XO-1 system by its gravitational tugs on XO-1b,” McCullough said. “It’s even possible that such a planet could be similar to Earth.”

Original Source: HubbleSite News Release

Artist illustration of a super Earth. Image credit: CfA. Click to enlarge.
Nearly all the extrasolar planets discovered have been Jupiter-sized or larger. But astronomers from the Harvard-Smithsonian Center for Astrophysics think that super-earths – rocky planets several times larger that our planet – might actually be much more common. Based on the recent discovery of a super-earth around a red dwarf star 9,000 light-years away, the research team calculated that there are probably 3 times as many of these planets than the larger gas giants.

Astronomers have discovered a new “super-Earth” orbiting a red dwarf star located about 9,000 light-years away. This newfound world weighs about 13 times the mass of the Earth and is probably a mixture of rock and ice, with a diameter several times that of Earth. It orbits its star at about the distance of the asteroid belt in our solar system, 250 million miles out. Its distant location chills it to -330 degrees Fahrenheit, suggesting that although this world is similar in structure to the Earth, it is too cold for liquid water or life.

Orbiting almost as far out as Jupiter does in our solar system, this “super-Earth” likely never accumulated enough gas to grow to giant proportions. Instead, the disk of material from which it formed dissipated, starving it of the raw materials it needed to thrive.

“This is a solar system that ran out of gas,” says Harvard astronomer Scott Gaudi of the Harvard-Smithsonian Center for Astrophysics (CfA), a member of the MicroFUN collaboration that spotted the planet.

The discovery is being reported today in a paper posted online at http://arxiv.org/abs/astro-ph/0603276 and submitted to The Astrophysical Journal Letters for publication.

Gaudi performed extensive data analysis that confirmed the existence of the planet. Further analysis simultaneously ruled out the presence of any Jupiter-sized world in the distant solar system.

“This icy super-Earth dominates the region around its star that, in our solar system, is populated by the gas giant planets,” said first author Andrew Gould (Ohio State University), who leads MicroFUN.

The team also calculates that about one-third of all main sequence stars may have similar icy super-Earths. Theory predicts that smaller planets should be easier to form than larger ones around low-mass stars. Since most Milky Way stars are red dwarfs, solar systems dominated by super-Earths may be more common in the Galaxy than those with giant Jupiters.

This discovery sheds new light on the process of solar system formation. Material orbiting a low-mass star accumulates into planets gradually, leaving more time for the gas in the protoplanetary disk to dissipate before large planets have formed. Low-mass stars also tend to have less massive disks, offering fewer raw materials for planet formation.

“Our discovery suggests that different types of solar systems form around different types of stars,” explains Gaudi. “Sun-like stars form Jupiters, while red dwarf stars only form super-Earths. Larger A-type stars may even form brown dwarfs in their disks.”

Astronomers found the planet using a technique called microlensing, an Einsteinian effect in which the gravity of a foreground star magnifies the light of a more distant star. If the foreground star possesses a planet, the planet’s gravity can distort the light further, thereby signaling its presence. The precise alignment required for the effect means that each microlensing event lasts for only a brief time. Astronomers must monitor many stars closely to detect such events.

Microlensing is sensitive to less massive planets than the more common planet-finding methods of radial velocity and transit searches.

“Microlensing is the only way to detect Earth-mass planets from the ground with current technology,” says Gaudi. “If there had been an Earth-mass planet in the same region as this super-Earth, and if the alignment had been just right, we could have detected it. By adding one more two-meter telescope to our arsenal, we may be able to find up to a dozen Earth-mass planets every year.”

The OGLE (Optical Gravitational Lensing Experiment) collaboration initially discovered the microlensed star in April 2005 while peering in the direction of the galactic center, where both foreground and background stars are widespread. OGLE identifies several hundred microlensing events per year, however only a small fraction of those events yield planets. Gaudi estimates that with one or two additional telescopes located in the southern hemisphere to monitor the galactic center, the planet count could jump drastically.

The discovery was made by 36 astronomers, including members of the MicroFUN, OGLE, and Robonet collaborations. The name of the planet is OGLE-2005-BLG-169Lb. OGLE-2005-BLG-169 refers to the 169th microlensing event discovered by the OGLE Collaboration toward the Galactic bulge in 2005, and “Lb” refers to a planetary mass companion to the lens star.

Crucial roles in the discovery were played by OGLE team leader Andrzej Udalski of Warsaw University Observatory and graduate students Deokkeun An of Ohio State and Ai-ying Zhou of Missouri State University. Udalski noticed that this microlensing event was reaching a very high magnification on May 1, and he quickly alerted the MicroFUN group to this fact, since high magnification events are known to be very favorable for planet detection. MicroFUN’s regular telescopes were unable to get many images, so MicroFUN leader Gould called the MDM Observatory in Arizona where An and Zhou were observing. Gould asked An and Zhou to obtain a few measurements of the star’s brightness over the course of the night, but instead An and Zhou made more than 1000 measurements. This large number of MDM measurements was crucial for the determination the observed signal must really be due to a planet.

Original Source: CfA News Release

Computed intensity of vortex coronagraph for a single point-like source. Image credit: Grover Swartzlander. Click to enlarge
“Some people say that I study darkness, not optics,” jokes Grover Swartzlander.

But it’s a kind of darkness that will allow astronomers to see the light.

Swartzlander, an associate professor in The University of Arizona College of Optical Sciences, is developing devices that block out dazzling starlight, allowing astronomers to study planets in nearby solar systems.

The devices also may prove valuable to optical microscopy and be used to protect camera and imaging systems from glare.

The core of this technology is an “optical vortex mask” – a thin, tiny, transparent glass chip that is etched with a series of steps in a pattern similar to a spiral staircase.

When light hits the mask dead on, it slows down more in the thicker layers than in thinner ones. Eventually, the light is split and phase shifted so some waves are 180 degrees out of phase with others. The light spins through the mask like wind in a hurricane. When it reaches the “eye” of this optical twister, light waves that are 180 degrees out of phase cancel one another, leaving a totally dark central core.

Swartzlander says this is like light following the threads of a bolt. The pitch of the optical “bolt” – the distance between two adjacent threads – is critical. “We’re creating something special where the pitch should correspond to a change in the phase of one wavelength of light,” he explained. “What we want is a mask that essentially cuts this plane, or sheet, of incoming light and curls it up into a continuous helical beam.”

“What we’ve found recently is knock-your-socks-off amazing from a theoretical point of view,” he added.

“Mathematically, it’s beautiful.”

Optical vortices are not a new idea, Swartzlander noted. But it wasn’t until the mid 1990s that scientists were able to study the physics behind it. That’s when advances in computer-generated holograms and high-precision lithography made such research possible.

Swartzlander and his graduate students, Gregory Foo and David Palacios, garnered media attention recently when “Optics Letters” published their article on how optical vortex masks might be used on powerful telescopes. The masks could be used to block starlight and allow astronomers to directly detect light from a 10-billion-times-dimmer planet orbiting the star.

This could be done with an “optical vortex coronagraph.” In a traditional coronagraph, an opaque disk is used to block a star’s light. But astronomers who are searching for faint planets near bright stars can’t use the traditional coronagraph because glare from starlight diffracts around the disk obscuring light reflected from the planet.

“Any small amount of diffracted light from the star is still going to overwhelm the signal from the planet,” Swartzlander explained. “But if the spiral of the vortex mask coincides exactly with the center of the star, the mask creates a black hole where there is no scattered light, and you’d see any planet off to the side.”

The UA team, which also included Eric Christensen from UA’s Lunar and Planetary Lab, demonstrated a prototype optical vortex coronagraph on Steward Observatory’s 60-inch Mount Lemmon telescope two years ago. They couldn’t search for planets outside our solar system because the 60-inch telescope isn’t equipped with adaptive optics that corrects for atmospheric turbulence.

Instead, the team took pictures of Saturn and its rings to demonstrate how easily such a mask could be used with a telescope’s existing camera system. A photo from the test is online at Swartzlander’s website, http://www.u.arizona.edu/~grovers.

Optical vortex coronagraphs could be valuable to future space telescopes, such as NASA’s Terrestrial Planet Finder (TPF) and the European Space Agency’s Darwin mission, Swartzlander noted. The TPF mission will use space-based telescopes to measure the size, temperature, and placement of planets as small as the Earth in the habitable areas of distant solar systems.

“We’re applying for grants to make a better mask – to really ramp this thing up to get better quality optics, Swartzlander said. “We can demonstrate this now in the lab for laser beams, but we need a really good-quality mask to get closer to what’s needed for a telescope.”

The big challenge is developing a way to etch the mask to get “a big fat zero of light” at its core, he said.

Swartzlander and his graduate students are doing numerical simulations to determine the proper pitch for helical masks at the desired optical wavelengths. Swartzlander has filed a patent for a mask that covers more than one wavelength, or color of light.

The U.S. Army Research Office and State of Arizona Proposition 301 funds support this research.

The Army Research Office funds basic optical sciences research, although Swartzlander’s work also has practical defense applications.

Optical vortex masks also could be used in microscopy to enhance the contrast between biological tissues.

Original Source: UA News Release

Artist’s concept of planet orbiting a star. Image credit: NASA Click to enlarge
A NASA-led team of astronomers have used NASA’s Spitzer Space Telescope to detect a strong flow of heat radiation from a toasty planet orbiting a nearby star. The findings allowed the team to “take the temperature” of the planet.

“This is the closest extrasolar planet to Earth that has ever been detected directly, and it presents the strongest heat emission ever seen from an exoplanet,” said Drake Deming of NASA’s Goddard Space Flight Center, Greenbelt, Md. Deming is the lead author of a paper on this observation to be published in the Astrophysical Journal on June 10. An advance copy of the paper will be posted on the astro-ph website on Feb. 22.

The planet “HD 189733b” orbits a star that is a near cosmic neighbor to our sun, at a distance of 63 light years in the direction of the Dumbbell Nebula. It orbits the star very closely, just slightly more than three percent of the distance between Earth and the sun. Such close proximity keeps the planet roasting at about 844 Celsius (about 1,551 Fahrenheit), according to the team’s measurement.

The planet was discovered last year by Francois Bouchy of the Marseille Astrophysics Laboratory, France, and his team. The discovery observations allowed Bouchy’s team to determine the planet’s size (about 1.26 times Jupiter’s diameter), mass (1.15 times Jupiter), and density (about 0.75 grams per cubic centimeter). The low density indicates the planet is a gas giant like Jupiter.

The observations also revealed the orbital period (2.219 days) and the distance from the parent star. From this distance and the temperature of the parent star, Bouchy’s team estimated the planet’s temperature was at least several hundred degrees Celsius, but they were not able to measure heat or light emitted directly from the planet.

“Our direct measurement confirms this estimate,” said Deming. This temperature is too high for liquid water to exist on the planet or any moons it might have. Since known forms of life require liquid water, it is unlikely to have emerged there.

Last year, Deming’s team and another group based at the Harvard-Smithsonian Center for Astrophysics used Spitzer to make the first direct detection of light from alien worlds, by observing the warm infrared glows of two other previously detected “Hot Jupiter” planets, designated HD 209458b and TrES-1.

Infrared light is invisible to the human eye, but detectable by special instruments. Some infrared light is perceived as heat. Hot Jupiter planets are alien gas giants that zip closely around their parent stars, like HD 189733b. From their close orbits, they soak up ample starlight and shine brightly in infrared wavelengths.

Deming’s team used the same method to observe HD 189733b. To distinguish the planet’s glow from its hot parent star, the astronomers used an elegant method. First, they used Spitzer to collect the total infrared light from both the star and its planet. Then, when the planet dipped behind the star as part of its regular orbit, the astronomers measured the infrared light coming from just the star. This pinpointed exactly how much infrared light belonged to the planet. Under optimal circumstances this same method can be used to make a crude temperature map of the planet itself.

“The heat signal from this planet is so strong that Spitzer was able to resolve its disk, in the sense that our team could tell we were seeing a round object in the data, not a mere point of light,” said Deming. “The current Spitzer observations cannot yet make a temperature map of this world, but more observations by Spitzer or future infrared telescopes in space may be able to do that.”

Deming’s team includes Joseph Harrington, Cornell University, Ithaca, N.Y.; Sara Seager, Carnegie Institution of Washington; and Jeremy Richardson, NASA Postdoctoral Fellow at Goddard, in the Exoplanets and Stellar Astrophysics Laboratory.

NASA’s Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for the agency’s Science Mission Directorate. Science operations are conducted at the Spitzer Science Center at Caltech. JPL is a division of Caltech.

Original Source: NASA News Release

An artist’s illustration shows the extrasolar planet orbiting a dim star. Image credit: NASA Click to enlarge
Using a relatively new planet-hunting technique that can spot worlds one-tenth the mass of our own, researchers have discovered a potentially rocky, icy body that may be the smallest planet yet found orbiting a star outside our solar system.

The discovery suggests the technique, gravitational microlensing, may be an exceptional technology for finding distant planets with traits that could support life.

“This is an important breakthrough in the quest to answer the question ‘Are we alone?'” said Michael Turner, assistant director for the National Science Foundation (NSF) mathematical and physical sciences directorate. “The team has discovered the most Earth-like planet yet, and more importantly, has demonstrated the power of a new technique that is sensitive to detecting habitable planets. It can probe a much greater portion of our galaxy and is complementary to other techniques.”

Located more than 20,000 light years away in the constellation Sagittarius, close to the center of our Milky Way galaxy, planet OGLE-2005-BLG-390Lb is approximately five-and-a-half times the mass of Earth.

Orbiting a star one-fifth the mass of the sun at a distance almost three times that of Earth’s orbit, the newly discovered planet is frigid: the estimated surface temperature is -364 degrees Fahrenheit (-220 degrees Celsius).

Although astronomers doubt this cold body could sustain organisms, researchers believe gravitational microlensing will bring opportunities for observing other rocky planets in the “habitable zones” of stars – regions where temperatures are perfect for maintaining liquid water and spawning life.

The discovery, authored by 73 collaborators from 32 institutions, appears in the Jan. 26 issue of the journal Nature.

OGLE (Optical Gravitational Lensing Experiment) project telescopes first observed the lensing event on July 11, 2005. In an attempt to catch microlensing events as they occur, OGLE scans most of the central Milky Way each night, discovering more than 500 microlensing events per year. But to detect the signature of low-mass planets, astronomers must observe these events much more frequently than OGLE’s one survey per night.

So, when OGLE detected the July 11 lensing, its early warning system alerted fellow astronomers across the globe to microlensing event OGLE-2005-BLG-390 (for the 390th galactic bulge OGLE discovered in 2005). At that point, though, no one knew a planet would emerge.

“The only way to realize the full scientific benefit of our observations is to share the data with our competition,” said co-author Bohdan Paczynski of Princeton University, who along with Andrzej Udalski of Warsaw University Observatory and their colleagues co-founded OGLE in 1997.

The telescopes of PLANET (Probing Lensing Anomalies NETwork) and RoboNet tracked the July 11 episode to completion, providing the data that confirmed the presence of a previously unknown planet. These telescopes collect observations more frequently in an attempt to detect the microlensing signature of planets.

“This discovery was possible because the sun never rises on the PLANET collaboration,” said lead author and PLANET researcher Jean-Philippe Beaulieu of the Institut d’Astrophysique de Paris, France. “The global nature of the PLANET collaboration was crucial for obtaining data throughout the 24-hour planetary signal,” he added.

Ironically, when preparing the final report, the researchers discovered that during its test runs, the new MOA (Microlensing Observations in Astrophysics) telescope, MOA-2, had taken additional measurements of the lensing event. The 6-foot (1.8-meter) aperture telescope has a wider field-of-view than the OGLE telescope, enabling it to observe 100 million stars many times per night. MOA-2 is one of several recent and future advancements that gravitational microlensing proponents hope will greatly increase the number of Earth-like planet discoveries.

OGLE also has plans to increase the field-of-view of its own telescope, and other microlensing groups are proposing to build a new telescope in South Africa. They have also proposed a space mission to see planets as small as Mars as well as free-floating planets that no longer orbit a host star.

“The new discovery provides a strong hint that low-mass planets may be much more common than Jupiters,” said co-author and PLANET researcher David Bennett of the University of Notre Dame. Most extrasolar planets found so far have been Jupiter-sized.

“Microlensing should have discovered dozens of Jupiters by now if they were as common as these five-Earth-mass planets. This illustrates the primary strength of the gravitational microlensing method: its ability to find planets of low-mass,” Bennett said.

Low-mass planets can yield signals that are too weak to detect with other methods. With microlensing, the signals of low-mass planets are rare but not weak. Thus, the rate of low-mass planet discoveries should increase dramatically if more microlensing events can be searched for planetary signals.

Original Source: NSF News Release

Artist illustration of a planet orbiting a very young, active star. Image credit: UFL. Click to enlarge.
Astronomers have discovered a planet orbiting a very young star nearly 100 light years away using a relatively small, publicly accessible telescope turbocharged with a new planet-finding instrument.

The feat suggests that astronomers have found a way to dramatically accelerate the pace of the hunt for planets outside our solar system.

“In the last two decades, astronomers have searched about 3,000 stars for new planets,” said Jian Ge, a professor of astronomy at the University of Florida. “Our success with this new instrument shows that we will soon be able to search stars much more quickly and cheaply ? perhaps as many as a couple of hundred thousand stars in the next two decades.”

Ge and colleagues at the University of Florida, Tennessee State University, the Institute of Astrophysics in Spain’s Canary Islands, Pennsylvania State University and the University of Texas presented their findings today at the American Astronomical Society’s annual meeting in Washington, D.C.

Their work is important in part because of what the astronomers found ? a planet, at least half as massive as Jupiter, orbiting a star just 600 million years old. That’s very young compared, for example, with the sun’s 5 billion years.

“This is one of the youngest stars ever identified with a planetary companion,” Ge said. Perhaps more significant, the instrument used to find the planet points the way to a much more accessible method for finding others ? including those capable of supporting life.

Planets outside our solar system are typically swamped by the light of their stars, making it difficult to observe them visually. In the 1990s, astronomers began using a measurement technique called Doppler radial velocity to detect planets by observing the wobble in a star that is gravitationally induced by an orbiting planet.

This technique, which has uncovered the vast majority of the 160-plus extrasolar planets found so far, works by hunting through the spectrum of starlight for the subtle Doppler shifts that occur as the star and planet move toward and away from their common center of mass. The instrument at the heart of this technique is usually a spectrograph, but this instrument is problematic.

“A major problem with spectrographs is that they collect only a small percentage of photons from the target light source, which means that they are only useful to search for distant planets when mounted on relatively large telescopes,” Ge said.

The astronomers’ new instrument, the Exoplanet Tracker, or ET, eliminates this problem by swapping the spectrograph with an interferometer, a device that can take more precise radial velocity measurements. Tests show the interferometer can capture as much as 20 percent of available photons, making the instrument far more powerful, which opens its use for distant planet hunting to smaller telescopes.

At a development cost of about $200,000, the interferometer-equipped ET is also far cheaper than comparable spectrographs, which cost more than $1 million. And at about 4 feet long, 2 feet wide and weighing about 150 pounds, it is lighter and smaller. The instrument is based on a concept first proposed in 1997 by Lawrence Livermore National Lab physicist David Erskine.

The astronomers used the Exoplanet Tracker on the special 0.9-meter Coud? feed system within the National Science Foundation’s 2.1-meter telescope at Kitt Peak National Observatory near Tucson, Ariz.

Like radial velocity instruments equipped with spectrographs, the ET instrument in its present form can search only one object at a time. But Ge’s team has demonstrated that it can hunt for planets around multiple stars simultaneously ? a key element of its heightened utility. The team is working on a version capable of surveying as many as 100 stars simultaneously.

The Exoplanet Tracker will be used next spring for a trial planet survey on the Sloan Digital Sky Survey 2.5 meter wide-field telescope at the Apache Point Observatory in New Mexico. The new instrument is funded with an $875,000 grant from the W.M. Keck Foundation. A much more ambitious, long-term survey is in the planning stages.

The Kitt Peak Coud? feed telescope that Ge and colleagues used to discover the new planet has a 0.9-meter mirror on a tall tower, a mirror that directs incoming starlight into an observing room in the base of the 2.1-meter telescope. The standard spectrograph in the facility fills the room ? while ET occupies a small corner.

The new planet is the most distant ever found using the Doppler technique with a telescope mirror less than 1 meter in size. There are hundreds of such telescopes worldwide, compared with just a handful of the larger 2- and 3-meter telescopes more commonly used in planet finding ? telescopes that tend to be in extremely high demand and difficult to access.

“These smaller telescopes are relatively cheap and relatively available,” Ge said, “so you can often get access to many dozens of nights on them if you have a promising proposal.”

Kitt Peak National Observatory is part of the National Optical Astronomy Observatory, Tucson, Ariz., which is operated by the Association of Universities for Research in Astronomy Inc., under a cooperative agreement with the National Science Foundation.

“This is the first time that a planet has been discovered using a publicly funded telescope at the U.S. national observatory,” said Buell Jannuzi, acting director of Kitt Peak National Observatory. “We are very excited that the broader community of astronomers around the world will be able to propose to use the single-object Exoplanet Tracker instrument at Kitt Peak to carry out their own research programs, starting in the fall of 2006.”

That said, discovering new planets is never easy.

In the latest find, astronomers went to great lengths to ensure they were actually “seeing” a planet. That’s because the star, which has about 80 percent of the mass of our sun, retains much of its youthful rotation speed, which makes it capable of generating strong magnetic fields and associated dark star spots. These are similar to the magnetically generated sunspots on our own sun, and they can mimic the presence of a planet in orbit around the star.

To check against this possibility, Greg Henry, an astronomer at Tennessee State, observed the star with an automated telescope in Arizona, and found the star to be changing its brightness as it rotates.

“My observations reveal a rotation period of about 12 days for the star,” Henry said. “Thus, if the planetary orbital period is indeed less than five days, the dark spots rotating around on the surface of the star every 12 days cannot be causing the false appearance of a planet.”

Located in the direction of the constellation Virgo, the newly discovered planet completes its orbit in less than five days, meaning it orbits very close to its parent star and is very hot. That means it’s too close to the star to lie within the “habitable zone” where life is possible.

Original Source: UFL News Release

An artist’s concept of a debris disk forming planets. Image credit: NASA/JPL Click to enlarge
Astronomers have found a debris disk around a sun-like star that may be forming or has formed its terrestrial planets. The disk – a probable analog to our asteroid belt – may have begun a solar-system-scale demolition derby, where the rocky remains of failed planets collide chaotically.

“This is one of a very rare class of objects that may give us a glimpse into what our solar system may have looked like during the formation of our terrestrial planets,” said Dean C. Hines of the Space Science Institute, a leader of the team that discovered the rare objects with NASA’s Spitzer Space Telescope.

“The target is essentially a star similar to our sun, seen at a time when the terrestrial planets in our solar system were thought to have formed,” Hines said. “We see evidence that this star might have an asteroid belt, roughly at the distance Jupiter is from our sun.”

“This object is very unusual in the context of all the others we’ve looked at,” said University of Arizona assistant astronomy Professor Michael R. Meyer, a colleague in the discovery. Meyer directs a Spitzer Legacy project to study solar system formation and evolution in a sample of 328 young sun-like stars in the Milky Way. The project turned up the unusual system.

“This is the only such debris disk among the 33 sun-like stars we’ve studied in our project so far, and one of only five such objects known,” Meyer said.

The star, named HD 12039, is about 30 million years old, or the age of the sun when the terrestrial planets are thought to have been 80 percent complete and the Earth-moon system formed, the astronomers said. It is roughly 137 light years away, or the distance light travels in 137 years.

HD 12039 is a “G” type star like our sun, a yellow star with surface temperatures between 5,000 and 7,000 degrees Fahrenheit. It hasn’t yet settled into the “main sequence,” or mature nuclear-burning phase as our sun has. It’s eight percent brighter, just slightly cooler and a little more massive than our sun, or 1.02 solar masses.

The Spitzer team discovered that the star’s debris disk temperature is 110 degrees Kelvin, or minus 262 degrees Fahrenheit. That’s warmer than temperatures of the frigid outer debris disks that Meyer’s Spitzer team commonly finds around sun-like stars. They’ve found that between 10 and 20 percent of the sun-like stars in their sample so far — whether young, middle-aged or old — have outer disks like our Kuiper Belt beyond Neptune.

“The temperature of the dust in HD 12039’s strange, narrow debris ring puts it between four and six astronomical units from the star — smack dab where Jupiter is in our solar system,” Meyer said. (An astronomical unit, or AU, is the mean distance between Earth and the sun.)

“What’s curious about this disk is that there’s little if any dust inside four AU and beyond six AU. It’s a narrowly confined ring that could be similar in some ways to the outer rings we see around Saturn,” Meyer said.

Just as small moons shepherd the ice grains orbiting Saturn into discrete rings, and just as Jupiter tends the outer edge of our solar system’s asteroid belt, an unseen giant planet may be nudging dust into the narrow debris ring around this star, the astronomers said.

“We think this is a tight, narrow ring of rocky objects similar to those in our asteroid belt, except this ring is five AU from its star, instead of two-to-three AU, the distance between our asteroid belt and the sun,” Meyer said.

“At 30 million years, the material we see in this star likely has to come from ground-up rocks in a zone where terrestrial planets could form,” Hines said.

NASA earlier this year announced a Spitzer telescope discovery of another of these alien asteroid belts. It orbits a two-billion-year-old sun-like star 35 light years away, at a distance comparable to that between Venus and the sun.

Based on Spitzer Telescope results to date, only one percent to three percent of the young, sun-like stars in our Milky Way have massive terrestrial debris disks, Meyer said.

“We could be witnessing a common, short-lived event through which all systems pass, or we could be seeing a rare example of a massive warm debris disk surrounding an unusual, sun-like star,” Meyer said.

The astronomers describe their work in an article to be published in The Astrophysical Journal.

The Jet Propulsion Laboratory manages the Spitzer Space Telescope mission for NASA’s Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at Caltech. Caltech manages JPL for NASA. For information on the Spitzer Space Telescope visit:
http://www.spitzer.caltech.edu/spitzer

The Space Science Institute is a nonprofit organization that carries out world-class research in space and Earth science, together with innovative science education programs that inspire and deepen the public’s understanding of planet Earth and its place in the grander universe. The institute’s integrated research and education programs span planetary science, space physics, astrophysics, astrobiology and Earth science.

Original Source: UA News Release

***image***Astronomers looking for earth-like planets in other solar systems ? exoplanets ? now have a new field guide thanks to earth and planetary scientists at Washington University in St. Louis.

Bruce Fegley, Ph.D., Washington University professor of earth and planetary sciences in Arts & Sciences, and Laura Schaefer, laboratory assistant, have used thermochemical equilibrium calculations to model the chemistry of silicate vapor and steam-rich atmospheres formed when earth-like planets are undergoing accretion . During the accretion process, with surface temperatures of several thousands degrees Kelvin (K), a magma ocean forms and vaporizes.

“What you have are elements that are typically found in rocks in a vapor atmosphere,” said Schaefer. “At temperatures above 3,080 K, silicon monoxide gas is the major species in the atmosphere. At temperatures under 3,080 K, sodium gas is the major species. These are the indicators of an earth-like planet forming.”

At such red-hot temperatures during the latter stages of the exoplanets’ formation, the signal should be distinct, said Fegley.

“It should be easily detectable because this silicon monoxide gas is easily observable,” with different types of telescopes at infrared and radio wavelengths, Fegley said.

Schaefer presented the results at the annual meeting of the Division of Planetary Sciences of the American Astronomical Society, held Sept. 4-9 in Cambridge, England. The NASA Astrobiology Institute and Origins Program supported the work.

Forming a maser

Steve Charnley, a colleague at NASA AMES, suggested that some of the light emitted by SiO gas during the accretion process could form a maser ? Microwave Amplification by Stimulation Emission of Radiation. Whereas a laser is comprised of photons in the ultraviolet or visible light spectrum, masers are energy packets in the microwave image.

Schaefer explains: “What you basically have is a clump of silicon monoxide gas, and some of it is excited into a state higher than ground level. You have some radiation coming in and it knocks against these silicon monoxide molecules and they drop down to a lower state.

“By doing that, it also emits another photon, so then you essentially have a propagating light. You end up with this really very high intensity illumination coming out of this gas.”

According to Schaefer, the light from newly forming exoplanets should be possible to see.

“There are natural lasers in the solar system,” she said. “We see them in the atmospheres of Mars and Venus, and also in some cometary atmospheres.”

In recent months, astronomers have reported earth-like planets with six to seven times the mass of our earth. While they resemble a terrestrial planet like earth, there has not yet been a foolproof method of detection. The spectra of silicon monoxide and sodium gas would be the indication of a magma ocean on the astronomical object, and thus an indication a planet is forming, said Fegley.

The calculations that Fegley and Schaefer used also apply to our own earth. The researchers found that during later, cooler stages of accretion (below 1,500 K), the major gases in the steam-rich atmosphere are water, hydrogen, carbon dioxide, carbon and nitrogen, with the carbon converting to methane as the steam atmosphere cools.

Original Source: WUSTL News Release