September 19, 2008

Image of a Possible Exoplanet

Photo credit: Gemini Observatory

There's a very interesting image of a possible exoplanet around a sun-like star in the constellation Scorpius. This is noteworthy because no other exoplanet has ever been observed directly near such a bright star. Usually the starlight is so brilliant - and the planet(s) so close to the star - that the planet cannot be observed directly. Instead, exoplanets are normally detected either by the gravitational pull they exert on their star or they orbit very dim brown dwarf stars that don't hide the reflected light from the planet in their own brilliance.

Whether or not the planet in the photo is indeed gravitationally tied to the star in the photo can't be determined for another two years. This is due to the fact that the exoplanet is so far away from its star: 330 AU (astronomical units; the distance from the earth to the sun is 1 AU). This is
really distant! To compare it against the known dwarf planets orbiting our sun, the only one with a larger orbit is Sedna, which has an average distance from our sun of 486 AU.

Three University of Toronto scientists used the Gemini North telescope on Mauna Kea in Hawai'i to take images of 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 submitted to the Astrophysical Journal Letters and also posted online. “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 Ray Jayawardhana, team member and author of a forthcoming book on extrasolar planets entitled Worlds Beyond.

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. Taken together, such findings confirm that it is a very young, very low-mass object at roughly the same distance from Earth as the star.

Even though the likelihood of a chance alignment between such an object and a similarly young star is rather small, 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.

HT: Astronomy Picture of the Day

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