First image of planet around Sunlike star
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: September 16, 2008

Using the Gemini North telescope on Mauna Kea, astronomers have taken what is likely the first picture of a planet around a ‘normal’ star similar to our Sun.

The young star, 1RXS J160929.1-210524, which has about 85 percent the mass of our Sun, lies about 500 light years from Earth with a companion body bearing a mass about eight times that of Jupiter. The candidate planet lies 330 times the Earth-Sun distance away from its star, over ten times the distance that Neptune orbits the Sun in our Solar System.

"This is the first time we have directly seen a planetary mass object in a likely orbit around a star like our Sun," says David Lafrenicre, lead author of the paper that describes the findings in Astrophysical Journal Letters. "If we confirm that this object is indeed gravitationally tied to the star, it will be a major step forward."

Gemini near infrared adaptive optics image of 1RSX J160929.1-210524 (centre) and its likely ~8 Jupiter-mass companion (within red circle, top left). All images were obtained with the Gemini Altair adaptive optics system and the Near-Infrared Imager on the Gemini North telescope. Image: Gemini Observatory

Until now, the only planet-like bodies that have been directly imaged outside of the Solar System are either brown dwarfs, which are dim and make it easier to detect planetary-mass companions, or are not found orbiting a star at all. The near-infrared images and spectra of the suspected planetary object indicate that it is too cool to be a star or a brown dwarf, and that it is young. But the existence of a planetary-mass companion at such great distance 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," says team member Ray Jayawardhana.

The traditional formation mechanism for giant planets is known as core accretion, in which a small planet seed gradually grows by accreting solid particles from the circumstellar disc until this core becomes massive enough to capture large amounts of gas from the disc, at which point it becomes a giant planet. At hundreds of astronomical units from the star, as is the case for the newly discovered system, the density of material in the disc is so low that any small seed of planet would not be able to grow enough before the disc vanishes, which generally occurs after a few million years.

“So if this planet formed through this mechanism, it would have had to form at a small distance and then migrate outward to its current position,” explains Lafrenicre. “Outward migration can result from gravitational interaction between multiple planets, that is, the gravitational effects of the planets on each other modify their orbits, gradually sending one planet on a larger orbit, or from interaction of the planet with the circumstellar disc.”

Another possibility is that the planet formed directly at its current location but through a different mechanism. “Maybe it formed as binary stars do, by the direct collapse and fragmentation of a molecular cloud core, or maybe it formed by the rapid gravitational collapse of a part of the circumstellar disc,” suggests Lafrenicre. “All possibilities present their share of difficulties though, and it is hard to determine which one is more likely.”

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. “We need to obtain more images of the two objects to measure their relative positions as a function of time, and see if both objects are traveling through space together,” Lafrenicre tells Astronomy Now. “If the candidate companion is indeed 'tied' to the primary by gravity, then its position relative to the primary should be constant with time. If on the other hand it is not, then both objects would be moving through space at different speeds and in different directions, and their relative positions would thus change with time. Of course it would be premature to say that the object is definitely orbiting this star, but the evidence is extremely compelling.”

The work that led to this discovery is part of a survey of more than 85 stars in the Upper Scorpius association, a group of young stars formed about five million years ago. It uses the Gemini telescope's high-resolution adaptive optics capabilities to determine the different types of companions that can form around young stars: stars, brown dwarfs, or planetary mass objects. Young stars are good subjects since any planetary mass object they host would not have had time to cool and thus would still be relatively bright, allowing astronomers to more readily discover such objects. Indeed, the Jupiter-sized body has an estimated temperature of about 1500 degrees Celsius, much hotter than our own Jupiter, which has a temperature of about -110 deg C.

"This discovery certainly has us looking forward to what other surprises nature has in stock for us," says team member Professor Marten Van Kerkwijk.