SYDNEY: Astronomers have revealed one of the strangest planets yet discovered. The extrasolar oddity is 13 times the size of Jupiter and is so close to its star that it completes an orbit in just 3.2 days.
Unveiled this week at a meeting of the American Astronomical Society in Honolulu, Hawaii, XO-3b is one of 28 new exoplanets and nine other possible ‘brown dwarfs’ discovered in the last 12 months.
The planet was found by the XO Project, a collaboration between researchers and amateur astronomers; who help to clock up detailed observations of potential planetary systems over time.
“Of the 240-odd exoplanets found so far, XO-3b is an oddity in several respects,” said Peter McCullough, an XO Project astronomer at the Space Telescope Science Institute in Baltimore, USA.
“It’s the largest and most massive planet yet found in such a close orbit and, given the proximity of the orbit to the star, we were surprised to find that the orbit is not circular but significantly elliptical,” he said.
Normally, orbits of objects in close proximity to stars become circular with time, said McCullough. The egg-shaped elliptical orbit suggests XO-3b has a near neighbour tugging it out of a circular path, he said. Alternatively, astronomers could be underestimating the time it takes for planetary orbits to become circular.
There is a further mystery surrounding XO-3b, said XO Project astronomer Christopher Johns-Krull of Rice University in Houston, Texas. “We are intrigued that its mass is on the boundary between planets and brown dwarfs.”
Johns-Krull said there is still a lively debate among astronomers about how to classify so-called ‘brown dwarfs’. Technically, any stellar mass large enough to fuse hydrogen – which means it has a mass around 80 times that of Jupiter – is classified a star. Brown dwarfs are massive objects that just fall short of that mark, he said.
Most astronomers believe brown dwarfs – sometimes called ‘failed stars’ – form in a star-like manner. They suggest that early in their life cycle they undergo deuterium fusion, but not hydrogen fusion as larger stars do. Controversy arises when considering smaller brown dwarfs, which may never have sustained even deuterium fusion.
The International Astronomical Union – the same body that stripped Pluto of its planet status last year – considers any object larger than 13 times the size of Jupiter, which does not fuse hydrogen, to be a brown dwarf.
So, if XO-3b is classified as a planet instead, it will the largest planet ever discovered.
XO-3b was discovered because it is a ‘transiting planet’, this means its orbit passes across the face of its star as seen from Earth. Most exoplanets have been discovered by virtue of the way their gravitational pull causes their stars to wobble.
Fewer than two dozen transiting planets have been identified and XO-3b is the third found by the XO Project, which is specifically tasked with discovering them. Transiting planets are useful because their mass and radius are more easily estimated compared with non-transiting exoplanets.
Improved detection methods mean that one eighth of all exoplanets known were discovered in the past year, and that list is expanding rapidly.
“We’re just now getting to the point where, if we were observing our own solar system from afar, we would be seeing Jupiter,” commented astronomer Jason Wright of the University of California in Berkeley.