Astronomers discover planet 8-times larger than Jupiter

Astronomers have discovered a giant planet, at least eight times larger than Jupiter, that orbits a 2 million-year-old star about 450 light years from Earth, contradicting a long-standing idea that larger planets take longer to form.

The star CI Tau is so young that it still retains a disk of circumstellar gas and dust, researchers said.

"For decades, conventional wisdom held that large Jupiter-mass planets take a minimum of 10 million years to form," said Christopher Johns-Krull from Rice University in the US.

"That's been called into question over the past decade, and many new ideas have been offered, but the bottom line is that we need to identify a number of newly formed planets around young stars if we hope to fully understand planet formation," said Johns-Krull.

The planet CI Tau b is at least eight times larger than Jupiter and orbits the 2 million-year-old star about 450 light-years from Earth in the constellation Taurus, said researchers, including those from the University of Texas at Austin, NASA and Northern Arizona University.

Earth and the Sun are more than 4 billion years old, and while the 3,300-plus catalogue of exoplanets includes some older and some younger than Earth, the obstacles to finding planets around newly formed stars are varied and daunting, Johns-Krull said.

There are relatively few candidate stars that are young enough, bright enough to view in sufficient detail with existing telescopes and still retain circumstellar disks of gas and dust from which planets form.

Stars so young also are often active, with visual outbursts and dimmings, strong magnetic fields and enormous starspots that can make it appear that planets exist where they do not.

CI Tau b orbits the star CI Tau once every nine days. The planet was found with the radial velocity method, a planet-hunting technique that relies upon slight variations in the velocity of a star to determine the gravitational pull exerted by nearby planets that are too faint to observe directly with a telescope.

The discovery resulted from a survey begun in 2004 of 140 candidate stars in the star-forming region Taurus-Auriga.

"This result is unique because it demonstrates that a giant planet can form so rapidly that the remnant gas and dust from which the young star formed, surrounding the system in a Frisbee-like disc, is still present," said Lisa Prato of Lowell Observatory in the US.

"Giant planet formation in the inner part of this disk, where CI Tau b is located, will have a profound impact on the region where smaller terrestrial planets are also potentially forming," said Prato.

Johns-Krull said the team has examined about half of the young stars in the Taurus-Auriga survey sample, and the data from several of these suggest that more planets may be found.

The study was published in the Astrophysical Journal.