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First exo-planet discovered

08/05/2005. Contributed by Jessica Martin

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Astronomers confirm the first image of a planet outside of our solar system.

Among the most essential quests of modern astronomers, taking direct images of planets outside of our solar system is certainly up there among chart-toppers. Obtaining such images of a so-called exoplanet would enable scientists to study in detail the physical nature of the object and, in particular, to analyse the composition of its atmosphere. The astronomers' ultimate goal is of course to perform such analysis for earth-sized planets, in the hope of detecting a telltale signature of extraterrestrial life.

Such an ultimate objective is still at least decades in the future, as earth-size and even Jupiter-size planets around stars as old as the Sun are too faint to be detected by present-day technology.

Nevertheless, great progress can be achieved by taking images of giant planets orbiting much younger objects. Because giant planets a few tens of millions of years old are much hotter and brighter than their older brethren, they can be much more easily detected.

Moreover, as the first tens of millions of years are considered to have been a critical period in the formation of Earth and of our own solar system, the study of nearby young planetary systems provides astronomers with invaluable insight on our own origins, something that is difficult if not impossible to decipher from investigation of old, mature planetary systems.

Several surveys are therefore currently in progress that particularly aim at finding and taking direct images of sub-stellar companions - brown dwarfs and exoplanets - close to very young objects. Last year, an international team of astronomers [2] reported the first image of a Giant Planet Companion to 2M1207 (see ESO PR 23/04). On NACO/VLT images obtained in April 2004, they detected a faint reddish speck of light in the close vicinity of this young brown dwarf member of the 8 million year old TW Hydrae Association.

The feeble companion, now called 2M1207b, is more than 100 times fainter than the brown dwarf, 2M1207A. The spectrum of the companion presents the strong signature of water molecules. Based on the infrared colours and the spectral data, evolutionary model calculations lead to the conclusion that 2M1207b is a 5 Jupiter-masses planet [3]. Its mass can be estimated also by use of a different method of analysis which focuses on the strength of its gravitational field; this technique suggests that the mass might be even less than 5 Jupiter mass.

In April 2004, the spectroscopic and photometric analysis strongly indicated a planetary mass object close to the star. An alternative explanation, that the faint detected object was a background source unrelated to the young brown dwarf (such as an extragalactic object or a peculiar cool star with unusual infrared colours), appeared very unlikely.
Observations with the Hubble Space Telescope, obtained in August 2004 (i.e. only 4 months later), corroborated the VLT/NACO observations, even though they were obtained too soon after the NACO ones to provide a definite answer. Additional observations at a later epoch were required to prove beyond any doubt that the two objects, 2M1207A and 2M1207b, indeed move together in the sky and are therefore gravitationally bound to each other.

Such additional observations have now just been performed by the same team of European and American astronomers. They used again NACO on Yepun, the fourth 8.2-m Unit Telescope of ESO's Very Large Telescope [4]. The team took new images in February and March 2005 and measured the apparent motion on the sky of the young brown dwarf. For the three different epochs (April 2004, February and March 2005), they then accurately determined the relative position of the Giant Planet Companion with respect to the brown dwarf.

These observations show, with high accuracy, that there is no change in relative position between the two objects. This is exactly what one expects over a time scale of one year if 2M1207b is gravitationally bound to its host 2M1207A [5]. Over much longer time spans, we should see the two objects orbiting around each other.

For Gael Chauvin, astronomer at ESO and leader of the team of astronomers who conducted the study, "this new set of NACO measurements unambiguously confirms that 2M1207b is a planetary mass companion to the young brown dwarf 2M1207A. The image released last year is thus truly the first image ever taken of a planet outside of our solar system."

"The two objects - the giant planet and the young brown dwarf - are moving together; we have observed them for a year, and the new images essentially confirm our 2004 finding", says Benjamin Zuckerman, UCLA professor of physics and astronomy, member of NASA's Astrobiology Institute, and a member of the team. "I'm more than 99 percent confident. This would also be the first time that a planet outside of our solar system has been detected far from a star or brown dwarf - nearly twice as far as the distance between Neptune and the sun."

"Given the rather unusual properties of the 2M1207 system, the giant planet most probably did not form like the planets in our solar system," says Gael Chauvin. "Instead it must have formed the same way our Sun formed, by a one-step gravitational collapse of a cloud of gas and dust.

In the course of the same survey, the astronomers also discovered an interesting companion to the young star AB Pictoris of the 30 million years old Tucana-Horlogium Association located about 150 light years from Earth. This companion, imaged for the first time in March 2003, has a near-infrared luminosity and spectrum which points to a light and cool object.

Using the same strategy as for 2M1207b, the astronomers observed the AB Pic system at different epochs over a time span of a year and a half and confirmed that the companion is not a background object. Evolutionary model calculations point to a 13 to 14 Jupiter masses object with a temperature of ~1700 Kelvin. As the presently accepted separation between a high mass planet and a low mass brown dwarf is at 13.6 Jupiter masses [6], the newly discovered companion thus may lie at the exact boundary between these two classes of sub-stellar objects. It might therefore play the role of a unique "Rosetta stone" in the future. Remarkably, this companion is located very far from its host star - about 9 times further from AB Pictoris than Neptune is from the Sun. Nothing like this situation has ever been seen before in a planetary system.

Recently, a group of German astronomers presented evidence, based on VLT/NACO data, for a sub-stellar companion to the 1 million year old star GQ Lup (see ESO PR 09/05). However, the precise nature of this sub-stellar companion is still unknown as the very young age of the system makes comparison with theoretical models quite complicated. It was therefore only possible to assess with certainty that the close companion to GQ Lup has most probably a mass between 1 and 42 Jupiter-masses, and therefore could be either an exoplanet or a brown dwarf.

The discovery of 2M1207b, AB Pic b and GQ Lup b/B, all within a short period of time, bring evidence that new, carefully designed surveys, using state-of-art instruments on the most advanced facilities, can provide astronomers with images of planetary companions.

The first image of an exoplanet may now have been taken and confirmed and there can be little doubt that others will follow soon. The detailed study of a growing number of exoplanets with different masses and orbital properties will provide insight on theoretical formation models and afford a unique opportunity to learn more about how the solar system formed, hence about our own origins.

Anne-Marie Lagrange, another member of the team from the Grenoble Observatory (France), looks towards the future: "Our discovery represents a first step towards one of the most important goals of modern astrophysics: to characterize the physical structure and chemical composition of giant and, eventually, terrestrial-like planets."

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