Sunday 29 January 2012

A new study of XO-2b, a planet in a wide binary star system.

The planet XO-2b was the second planet discovered by the XO Telescope on Haleakala Volcano on Maui, Hawaii, in 2007. It orbits a K-type Orange Dwarf Star with a mass 98% of that of our sun, 480 light years from Earth in the Lynx Constellation. This star (XO-2) has a binary companion (XO-2S) of similar mass at a distance of about 4600 AU (4600 times the distance between the Earth an the sun), the two orbiting about their mutual centre of gravity. The planet itself was determine to have a mass 57% of that of Jupiter and 98% of its radius, and to orbit the star at a distance of 0.0639 AU (or 6.39% of the distance between the Earth and the Sun) every 63 hours. The planet has a similar radius to Jupiter despite it's much lower mass due to the heat it receives from the close-by star, which causes it to expand; it is much less dense than Jupiter as a consequence.

An artist's impression of XO-2b (right), with Jupiter for scale. Ignacio González Tapia.

The 25 December 2011 edition of the Publications of the Astronomical Society of Japan contains a paper by a team lead by Norio Narita of the National Astronomical Observatory of Japan, describing an attempt to find out if the binary companion effects the orbit of XO-2b. The team made additional observations of the XO-2 system using the Subaru 8.2 m Telescope on Mauna Kea and previously published data from the Fred Whipple Observatory in Arizona.

The team made a revised estimate of the size of the planet, coming up with 62% of the mass of Jupiter. They found that XO-2b has a very slight eccentricity and orbited in the plane of the system (rather than tilted at an angle), had a prograde orbit, and slowed down and speeded up slightly in the course of this orbit, indicative of the gravitational influence of another body in the system. However this perturbation could not easily be attributed to the presence of XO-2S, since it appears to be caused by a body with an orbital period of about 8 years, implying that it orbits at most about 4 AU from XO-2 (Jupiter obits our Sun at a distance of 5.2 AU, every 11 years). They concluded that there was probably a second planet in the XO-2 system. This does not preclude any influence of XO-2S on the orbit of XO-2b, but suggests that it is two small to have been detected by this study.