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First Ground-Based Detection Of Light From Transiting Exoplanets
by Staff Writers
Washington DC (SPX) Jan 19, 2009

This is an artist's impression of the star OGLE-TR-56 and its planet, as the planet is passing behind the star. Credit: D. Sing (IAP) / A and A.

Two independent groups have simultaneously made the first-ever ground-based detection of extrasolar planets thermal emissions. Until now, virtually everything known about atmospheres of planets orbiting other stars in the Milky Way has come from space-based observations.

These new results, accepted for publication in Astronomy and Astrophysics, open a new frontier to studying these alien worlds and are especially critical because the major space-based workhorse to these studies, the Spitzer telescope, will soon run out of cryogens, highly limiting its capabilities.

One team of scientists observed a planet named OGLE-TR-56b, which is a "hot Jupiter." Hot Jupiters are massive planets that orbit very close to their stars, whipping around them in 2 to 3 days. Since they are so close to their stars, they are believed to be hot enough to emit radiation in the optical and near-infrared wavelengths and be detectable from Earth.

The orbit of OGLE-TR-56b carries it behind its host star from the perspective of an observer on Earth, but a challenge to observing is that the planet is faint and in a crowded field, located in the direction of the center of our galaxy, about 5,000 light years away.

"Others have tried to detect planetary atmospheres from Earth, but to no avail," remarked co-author Mercedes Lopez-Morales at the Carnegie Institution's Department of Terrestrial Magnetism.

"We hit it right two nights last summer. The successful recipe is a planet that emits a lot of heat and has little to no wind in its atmosphere. Plus it has to be a clear, calm night on Earth to measure accurately the differences in thermal emissions when the planet is eclipsed as it goes behind the star.

Only about one of every 3,000 photons from the star comes from the planet. This eclipse allows us to separate the emissions of the planet from those of the star.

The magic moments came on July 2nd at the European Southern Observatory's Very Large Telescope (VLT) and on August 3rd on Carnegie's Magellan-Baade telescope in Chile." Lopez-Morales and colleague Sara Seager had earlier predicted that the ideal candidate for such a detection would be a planet with the characteristics of OGLE-TR-56b.

The scientists obtained over 600 images from both telescopes. "Because that part of the galaxy is so crowded and the planet so faint we needed these large telescopes," explained lead author David Sing from Institut d'Astrophysique de Paris. "The planet is glowing red-hot like a kitchen stove burner, but we had to know precisely when the eclipse was going to happen and measure the stellar flux very accurately so it could be removed to reveal the planet's thermal emission."

In the other study, published in the same issue of the journal, astronomers in the Netherlands detected thermal emission in the near-infrared from another exoplanet named TrES-3b, also from the ground. Information about atmospheres of hot Jupiters from Spitzer studies has helped both sets of scientists.

The hot Jupiters Spitzer has observed have similar atmospheric properties, in particular thermal inversions, in which a warm layer holds a cooler layer underneath. "OGLE-TR-56b is hotter than any that Spitzer has seen so far," said Lopez-Morales.

"At over 4400 degrees F it's the hottest atmosphere yet measured. It is way too hot for silicon or iron clouds to form, which would keep it dark-typical of the hot Jupiters that Spitzer had found. It's comforting to know that when Spitzer goes out of service, studies like these two will be able to keep the field alive."

First ground-based detection of light from transiting exoplanets
Transiting exoplanets are routinely detected when they pass in front of their parent star as viewed from the Earth, which only happens by chance.

The transit event causes a small drop in the observed starlight, which can then be detected. Fifty-five exoplanets have been detected this way since the observation of the first transiting planet HD 209458 b in 1999. When the planet revolves around its star or when it goes behind, the light coming from the system also varies, though the resulting smaller modulation is much harder to detect. This is mostly due to the small amount of light emitted by these exoplanets which are believed to be as dark as coal and reflect little of the incoming starlight.

Fortunately, some of these planets are very hot, thus emitting light, mostly at infrared wavelengths. Up to now, detections of this kind have only been made using the Spitzer infrared space telescope. This week, Astronomy and Astrophysics is publishing the two first ground-based detections of thermal emission from transiting, hot-Jupiter exoplanets, from two independent teams of astronomers that used different approaches.

One team includes Ernst De Mooij and Ignas Snellen (University of Leiden, Netherlands) who used the William Hershel 4.2 meter telescope in La Palma (Canary Islands, Spain) to observe the star TrES-3 and its planet TrES-3b. To be able to detect the light coming from the planet, they observed the planet exactly at the time when it passes behind the star.

They observed the event at infrared wavelengths, where the planet is at its brightest compared to the star (even if the planet is still much fainter than the star!) As they detected the light coming from the planet, they estimated the temperature of its atmosphere to about 2000 Kelvins. This indicates that the day side of the planet is extremely hot.

The other team, involving David Sing (IAP, France) and Mercedes Lopez-Morales (Carnegie Institution of Washington, USA), had a different approach. They looked at a much fainter star and its planet, OGLE-TR-56b. This planet is one of the most irradiated planets known so far, both because the planet is very close to the star and because the star is very hot.

To detect the slight modulation in light that occurs when the planet passes behind its star, they used the 8 meter Very Large Telescope (ESO, Chile) and the 6.5 meter Magellan Telescopes (Las Campanas, Chile) and were able to observe this event at visible wavelengths.

Indeed, the planet OGLE-TR-56b is heated so much by its star that it emits detectable amounts of light in the visible wavelengths, and not only in the infrared as TrES-3b does. Hence, Sing and Lopez-Morales measured the record-high temperature of a planetary atmosphere: 2700 Kelvins.

As in the case of TrES-3b, such a high day-side temperature indicates that winds cannot redistribute the heat efficiently from the day side to the night side.

These two independent results are very interesting for astronomers and planetary scientists because they allow a direct probe of the temperature of these planetary atmospheres, and because they show that such measurements can be made from ground-based observatories, and not only when using space telescopes.


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