Category: Extrasolar Planets

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The current exoplanet count — the number of planets astronomers have found orbiting other stars –stands at 312. That’s a lot of planets. But not a single one of them can be classified as Earth-like. We just don’t have the ability to detect planets that small yet. But it might help if we knew exactly where to look. New research using supercomputer simulations of dusty disks around sun-like stars show that planets nearly as small as Mars can create patterns in the dust that future telescopes may be able to detect. The research points to a new avenue in the search for habitable planets. “It may be a while before we can directly image earth-like planets around other stars but, before then, we’ll be able to detect the ornate and beautiful rings they carve in interplanetary dust,” says Christopher Stark, the study’s lead researcher at the University of Maryland, College Park.

Working with Marc Kuchner at NASA’s Goddard Space Flight Center in Greenbelt, Md., Stark modeled how 25,000 dust particles responded to the presence of a single planet — ranging from the mass of Mars to five times Earth’s — orbiting a sun-like star. Using NASA’s Thunderhead supercomputer at Goddard, the scientists ran 120 different simulations that varied the size of the dust particles and the planet’s mass and orbital distance.

“Our models use ten times as many particles as previous simulations. This allows us to study the contrast and shapes of ring structures,” Kuchner adds. From this data, the researchers mapped the density, brightness, and heat signature resulting from each set of parameters.

“It isn’t widely appreciated that planetary systems — including our own — contain lots of dust,” Stark adds. “We’re going to put that dust to work for us.”

Much of the dust in our solar system forms inward of Jupiter’s orbit, as comets crumble near the sun and asteroids of all sizes collide. The dust reflects sunlight and sometimes can be seen as a wedge-shaped sky glow — called the zodiacal light — before sunrise or after sunset.

The computer models account for the dust’s response to gravity and other forces, including the star’s light. Starlight exerts a slight drag on small particles that makes them lose orbital energy and drift closer to the star.

“The particles spiral inward and then become temporarily trapped in resonances with the planet,” Kuchner explains. A resonance occurs whenever a particle’s orbital period is a small-number ratio — such as two-thirds or five-sixths — of the planet’s.

For example, if a dust particle makes three orbits around its star every time the planet completes one, the particle repeatedly will feel an extra gravitational tug at the same point in its orbit. For a time, this extra nudge can offset the drag force from starlight and the dust can settle into subtle ring-like structures.

“The particles spiral in toward the star, get trapped in one resonance, fall out of it, spiral in some more, become trapped in another resonance, and so on,” Kuchner says. Accounting for the complex interplay of forces on tens of thousands of particles required the mathematical horsepower of a supercomputer.

Some scientists note that the presence of large amounts of dust could present an obstacle to directly imaging earthlike planets. Future space missions — such as NASA’s James Webb Space Telescope, now under construction and scheduled for launch in 2013, and the proposed Terrestrial Planet Finder — will study nearby stars with dusty disks. The models created by Stark and Kuchner give astronomers a preview of dust structures that signal the presence of otherwise hidden worlds.

“Our catalog will help others infer a planet’s mass and orbital distance, as well as the dominant particle sizes in the rings,” Stark says.

Stark and Kuchner published their results in the October 10 issue of The Astrophysical Journal. Stark has made his atlas of exo-zodiacal dust simulations available online.

Source: Goddard Space Flight Center

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What astronomers had expected to be a run-of-the-mill protoplanetary disk turned out to be evidence of a much more intriguing story. While observing the sun-like star BD 20 307, a team of astronomers noticed a large disk of dust surrounding the star. Usually, this is evidence of planetary formation around younger stars. The 8 planets (and plutoids…) in our own solar system formed out of just such a disk. Disks like this aren’t generally found around older stars, though, and when the age of the star was calculated to be several billion years old, the source of the dust appears to come from a rare event: it is the resulting debris of two planets slamming into each other.

Using data from the Chandra X-ray Observatory, and taking the brightness using one of Tennessee State University’s automated telescopes in Arizona, the team first discovered BD 20 307 to in fact be part of a close binary pair. Not only that, but the system was much older than previously thought: several billions of years old, rather than a few hundred million. The system is 300 light-years away from Earth in the constellation Ares.

The curiously large amount of dust orbiting BD 20 307 is 1 million times the amount of dust than is found in our own solar system, and orbits at a distance from the star that is similar to the orbits of Earth and Venus around our own Sun. The abundance of dust particles in this orbit – and around such a mature star – led scientists to the conclusion that it was created by the violent collision of two exoplanets.

Benjamin Zuckerman, UCLA professor of physics and astronomy and co-author of a paper on the discovery said, “It’s as if Earth and Venus collided with each other. Astronomers have never seen anything like this before. Apparently, major catastrophic collisions can take place in a fully mature planetary system.” Zuckerman and his team will report their findings in the December issue of the Astrophysical Journal.

Normally, warm disks of dust surround younger star systems, out of which larger and larger structures can form, eventually yielding planets. To find a disk of dust in around a star that is several billions of years old is odd, because the pressure of stellar radiation pushes out the lighter dust over time, and the larger chunks either form planets and asteroids, or break down in collisions and get blown away.

The collision between the planets took place within the past few hundred thousand years, though it is possible that it happened even more recently. Such a colossal collision raises the question of how the orbits of the two planets became destabilized, and whether such a collision could happen in our own solar system.

“The stability of planetary orbits in our own solar system has been considered for nearly two decades by astronomer Jacques Laskar in France and, more recently, by Konstantin Batygin and Greg Laughlin in the U.S.A. Their computer models predict planetary motions into the distant future and they find a small probability for collisions of Mercury with Earth or Venus sometime in the next billion years or more. The small probability of this happening may be related to the rarity of very dusty planetary systems like BD+20 307,” said paper co-author Gregory Henry, astronomer at Tennessee State University (TSU).

Source: EurekAlert

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Astronomers have unveiled what is likely the first picture of a planet around a normal star similar to the Sun. Using the Gemini North telescope on Mauna Kea in Hawaii, astronomers from the University of Toronto imaged the young star 1RXS J160929.1-210524, which lies about 500 light-years from Earth and a candidate companion of that star. They also obtained spectra to confirm the nature of the companion, which has a mass about eight times that of Jupiter, and lies roughly 330 times the Earth-Sun distance away from its star. For comparison, the most distant planet in our solar system, Neptune, orbits the Sun at only about 30 times the Earth-Sun distance. The parent star is similar in mass to the Sun, but is much younger. “This is the first time we have directly seen a planetary mass object in a likely orbit around a star like our Sun,” said David Lafrenière, lead author of a paper detailing the discovery. “If we confirm that this object is indeed gravitationally tied to the star, it will be a major step forward.”

Until now, the only planet-like bodies that have been directly imaged outside of the solar system are either free-floating in space (i.e. not found around a star), or orbit brown dwarfs, which are dim and make it easier to detect planetary-mass companions.

The existence of a planetary-mass companion so far from its parent star comes as a surprise, and poses a challenge to theoretical models of star and planet formation. “This discovery is yet another reminder of the truly remarkable diversity of worlds out there, and it’s a strong hint that nature may have more than one mechanism for producing planetary mass companions to normal stars,” said team member Ray Jayawardhana.

The team’s Gemini observations took advantage of adaptive optics technology to dramatically reduce distortions caused by turbulence in Earth’s atmosphere. The near-infrared images and spectra of the suspected planetary object indicate that it is too cool to be a star or even a more massive brown dwarf, and that it is young.

While it could be a chance alignment between the object and the young star, it will take up to two years to verify that the star and its likely planet are moving through space together. “Of course it would be premature to say that the object is definitely orbiting this star, but the evidence is extremely compelling. This will be a very intensely studied object for the next few years!” said Lafrenière.

Team member Marten van Kerkwijk described the group’s search method. “We targeted young stars so that any planetary mass object they hosted would not have had time to cool, and thus would still be relatively bright,” he said. “This is one reason we were able to see it at all.”

The Jupiter-sized body has an estimated temperature of about 1800 Kelvin (about 1500ºC), much hotter than our own Jupiter, which has a temperature of about 160 Kelvin (-110ºC), and its likely host is a young star of type K7 with an estimated mass of about 85% that of the Sun.

“This discovery certainly has us looking forward to what other surprises nature has in stock for us,” said Van Kerkwijk.

Read the team’s paper here.

Source: Gemini Observatory

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You gotta love this about science; someone is always checking your work. Early this year a new exoplanet discovery was announced: TW Hydrae b, a huge planet about ten times as massive as Jupiter. Astronomers thought the planet was in a super-tight orbit around its host star (TW Hydrae), circling in only 3.56 days at a distance of about 6 million kilometers, which is about 4 percent of the distance from the Sun to the Earth. However, another group of astronomers decided to analyze some new optical and infrared data to confirm the radial velocity signal of the planet. Something didn’t seem right, so they ran a few more tests and computer models and determined what they were seeing wasn’t a planet. It was a big sunspot. “Our model shows that a cold spot covering 7% of the stellar surface and located at a latitude of 54 deg can reproduce the reported RV variations,” the astronomers reported in their paper. The rest of the astronomical world must agree with the new determination, as TW Hydrae b has now been dropped from the Planet Quest New Worlds Atlas (a fun site to peruse.) But nature doesn’t like a void, — and astronomers have been working hard in the planet-search department, — so, three new extra solar planets have been discovered and added to the atlas, for a current planet count of 309.

GJ 832 b is about half the size mass of Jupiter and orbits 3.4 AU from its tiny host star. The star is a yellow, sun-like G star, about 16 light years from Earth. It was found with the Anglo-Australian Telescope. Astronomers say it has the largest angular distance from its star among radial velocity detected exoplanets, which makes it a potentially interesting target for future direct detection.

HD 205739 b was also just announced:. This exoplanet is 1.37 times the size mass of Jupiter, and orbits about .9 AU from its star, a blue to white star, which is 1.22x the size of the sun, and 294 light years from Earth. It has an eccentric orbit, and astronomers believe there may be an additional planet in this system, because of how the planet orbits.

Another planet found by the same astronomical team is HD 154672 b. This is a biggie, at about five times the size mass of Jupiter, but only about .6 AU distant from its star, which is just about sun-size, and about 213 light years from Earth. The planet has an orbital period of 163.9 days.

These last two planets were found using the N2K Doppler planet search program with the Magellan telescopes.

Sources: arXiv (here, here and here) and Twitter, PlanetQuest

Using a new technique with a near-infrared spectrograph attached to ESO’s Very Large Telescope, astronomers have been able to study planet-forming discs around young Sun-like stars in unsurpassed detail, clearly revealing the motion and distribution of the gas in the inner parts of the disc. Astronomers used a technique known as ‘spectro-astrometric imaging’ to give them a window into the inner regions of the discs where Earth-like planets may be forming. They were able not only to measure distances as small as one-tenth the Earth-Sun distance, but also measure the velocity of the gas at the same time. “This is like going 4.6 billion years back in time to watch how the planets of our own Solar System formed,” says Klaus Pontoppidan from Caltech, who led the research.

Pontoppidan and colleagues have analyzed three young analogues of our Sun that are each surrounded by a disc of gas and dust from which planets could form. These three discs are just a few million years old and were known to have gaps or holes in them, indicating regions where the dust has been cleared and the possible presence of young planets. However, each of the discs are very different from each other and likely will result in very different planetary systems. “Nature certainly does not like to repeat herself,” said Pontoppidan.

For one of the stars, SR 21, a massive giant planet orbiting at less than 3.5 times the distance between the Earth and the Sun has created a gap in the disc, while for the second star, HD 135344B, a possible planet could be orbiting at 10 to 20 times the Earth-Sun distance. Observations of the disc surrounding the third star, TW Hydrae, may indicate the presence of one or two planets.

The new results not only confirm that gas is present in the gaps in the dust, but also enable astronomers to measure how the gas is distributed in the disc and how the disc is oriented. In regions where the dust appears to have been cleared out, molecular gas is still highly abundant. This can either mean that the dust has clumped together to form planetary embryos, or that a planet has already formed and is in the process of clearing the gas in the disc.

CRIRES, the near-infrared spectrograph attached to ESO’s Very Large Telescope, is fed from the telescope through an adaptive optics module which corrects for the blurring effect of the atmosphere and so makes it possible to have a very narrow slit with a high spectral dispersion: the slit width is 0.2 arcsecond and the spectral resolution is 100 000. Using spectro-astrometry, an ultimate spatial resolution of better than 1 milli-arcsecond is achieved.

“The particular configuration of the instrument and the use of adaptive optics allow astronomers to carry out observations with this technique in a very user-friendly way: as a consequence, spectro-astrometric imaging with CRIRES can now be routinely performed,” says team member Alain Smette, from ESO.

Source: ESO Press Release

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The European Space Agency’s COROT spacecraft has discovered an unusual exoplanet orbiting a star slightly more massive than our Sun. The planet, currently called COROT-exo-4b, is about the same size as Jupiter, and it takes 9.2 days to orbit its star. Most peculiar however, is that the planet and the star are in sync: the star rotates at the same pace as the planet’s period of revolution. Astronomers feel the planet is too low in mass and too distant from the star for the star to have any major influence on the planet’s rotation. But they are trying to understand the special interaction between this star and planet.

COROT stands for Convection, Rotation and planetary Transits. Launched in 2006, the mission has now observed more than 50,000 stars. The spacecraft is designed to detect rocky exoplanets almost as small as Earth. The satellite uses transits, the tiny dips in the light output from a star when a planet passes in front of it, to detect and study planets. This is followed up by extensive ground-based observations.

COROT-exo-4b is the fifth exoplanet found by the COROT spacecraft. Monitoring continuously over several months, the team tracked variations in its brightness between transits. They derived its period of rotation by monitoring dark spots on its surface that rotated in and out of view. It takes 9.2 days for the planet to orbit its star, which so far, is the longest period for any transiting exoplanet ever found.

It is not known whether COROT-exo-4b and its star have always been rotating in sync since their formation about 1000 million years ago, or if the star’s rotation synchronized later. Studying such systems with COROT will help scientists gain valuable insight into star-planet interactions.

This is the first transiting exoplanet found with such a peculiar combination of mass and period of rotation. Astronomers believe there must be something unique about how it formed and evolved.

Original News Source: ESA

HD 189733b is a Jupiter-sized extrasolar planet orbiting a yellow dwarf star. Due to its size and compact orbit, HD 189733b is one of the most studied extrasolar planets. HD 189733b shares many similar characteristics as HD 209458b (a.k.a. “Osiris,” as I reported in a UT article yesterday), and similar techniques have been used to analyse the spectral emissions from both parent stars. Although HD 189733b’s atmosphere isn’t thought to be evaporating like Osiris’, atmospheric gases extend far beyond the planetary “surface” allowing stellar light to pass through, giving astronomers a peek into what chemical compounds surround HD 189733b. From this analysis, scientists have deduced that water and methane is contained in the atmosphere; the Spitzer space telescope has even mapped the temperature distribution around the globe. Now, an Indian researcher has published work indicating a thin layer of particles exists in the upper atmosphere of HD 189733b. So what is the weather like on HD 189733b?

HD 189733b was discovered in 2005 and orbits a star in a binary system called HD 189733 in the constellation of Vulpecula. As the main star in the binary is a variable star (due to the transit of HD 189733b, periodically eclipsing the star), it has been designated with the variable name V452 Vulpeculae. The star system itself is located near the Dumbell Nebula, approximately 62 light years from Earth. As the star is relatively dim, as the exoplanet transits the star, there is an appreciable decrease in luminosity (of about 3%), creating the ideal conditions for the atmosphere of HD 189733b to be studied.

This exoplanet is approximately the same mass (1.15 ± 0.04 M_J) and radius (1.154 ± 0.032 R_J) as Jupiter, but it orbits very close to its parent star (~0.03 AU) so it is known as a “Hot Jupiter.” Due to the water/methane mix in the planet’s atmosphere, it is believed HD 189733b may have a blue hue, much like the colour of Uranus.

In 2007, the Spitzer Space Telescope observed HD 189733b and compiled a temperature map of the planet, showing that the equator was much hotter than the poles. Astronomers were also able to deduce that the atmosphere contains iron, silicate and aluminium oxide particulates. In new research by Sujan Sengupta from the Indian Institute of Astrophysics in Bangalore, it appears that these particles may collect in the upper atmosphere, forming a thin haze. This tentative conclusion was reached after careful examination of the polarization of emission from the star as HD 189733b transited. Preliminary results suggest there is a thin, reflective cloud in the exosphere.

So what is the weather like on HD 189733b? Hot and cloudy.

Source: arXiv Blog
Paper: arXiv:0807.1794v1 [astro-ph]

Observations of planets orbiting other stars are becoming increasingly common as astronomical techniques become more and more sophisticated. But some extrasolar planets have a stronger than normal spectroscopic signature, often stronger than their optical signature. What could be causing this? In a recent study, observations of the extrasolar planet HD 209458b (also unofficially known as “Osiris”, which orbits a star in the constellation of Pegasus) revealed the strongest ever spectroscopic signature for a giant extrasolar planet, indicating Osiris is producing a huge cloud of gas. This gas is being lost from the planet’s atmosphere; Osiris is evaporating…

Osiris orbits a star (imaginatively) called HD 209458, a yellow dwarf not too dissimilar to our Sun (with 1.1 solar masses, 1.2 solar radii and a surface temperature of 6000 K). This extrasolar planet is special in that it is readily observable during its transit period of 3.5 terrestrial days. This very short year is due to its small orbital radius of only 0.047 AU. Osiris could be called a “hot Jupiter” as it is a gas giant, approximately 60% the mass of Jupiter and it orbits within 0.05 AU of its parent star. Because of its close proximity to HD 209458, Osiris has a surface temperature of over 1000 K.

Osiris’ size and compact orbit causes HD 209458’s luminosity to vary by 2% as the planet passes in front of the star. It is for this reason that HD 209458 has been designated as a “variable star” with the name V376 Pegasi.

However, spectroscopic analysis of the star show that emissions from elements such as neutral hydrogen and a carbon ion are dimmed far more than the 2% optical luminosity dimming. What could be causing this increase in dimming for spectroscopic emission lines? As light is produced by HD 209458, it is blocked by the Osiris planetary disk, creating the 2% dimming observed by optical instrumentation. However, something is increasing the disk cross section area, absorbing certain spectral wavelengths of stellar emission. For example, there is a 5-15% dimming effect on neutral hydrogen (H I at 121.6 nm) and a 7-13% dimming effect on both atomic oxygen (O I at 130.5 nm) and singly ionized carbon (C II at around 133.5 nm). This led astronomers to realize there was a cloud of gas surrounding Osiris, allowing most of the optical wavelengths to pass through, but absorbing some spectroscopic lines.

As Osiris is orbiting so close to its star, the X-ray and EUV emissions are exciting gases in the exosphere (the uppermost reaches of the gas giant’s atmosphere), causing heating and expansion. As the planet is strongly influenced by its star’s gravitational pull, tides will play a strong part in amplifying the expansion of Osiris’ atmosphere. At a certain point, when the planet’s “exobase” (or the base of the exosphere) reaches the Roche Limit, atmospheric gases will begin to escape the gravitational pull of the planet and the interaction with HD 209458 causes a geometrical blow-off, ejecting huge amounts of atmospheric gases into space. The atmosphere of Osiris is therefore evaporating.

This is an intriguing subject, and more details can be found in the review recently published by David Ehrenreich from the Laboratoire d’astrophysique de Grenoble, Universite Joseph Fourier, France.

Source: arXiv:0807.1885v1 [astro-ph]