Quartet of giant planets puzzles astronomers

The discovery of a fourth giant world around the star HR 8799 is straining the two leading theories of how planets form.

Planets are thought to coalesce from a dusty disc around a young star. One model, called core accretion, says that giant planets form when the dust gathers into a rocky core, which then draws in gas to form a massive atmosphere. Another, called disc instability, says that these planets collapse suddenly from sections of the disc.

HR 8799's four planets, each five to 13 times Jupiter's mass, are too far apart to be explained easily by either model, say Christian Marois of the Herzberg Institute of Astrophysics in Victoria, British Columbia, Canada, and colleagues.

"This is the widest range of orbital radii of any planetary system known," Marois told New Scientist. His team imagedMovie Camera three planets around the star in 2008 and has now found the fourth.

The outermost planet is nearly 70 times as far from its star as the Earth is from the sun. At that distance, dust moves so slowly that by the time it had snowballed into a rocky core, the star would already have blown away most of the gas in the disc around it. That would have prevented a giant planet from forming in the core accretion model, the researchers say.

The newly discovered innermost planet challenges the rival disc instability model. It orbits at 15 times the Earth-sun distance – where the star's heat would prevent the disc from collapsing, the researchers argue.

It is unlikely that a mixture of the two processes would have produced planets with such similar masses, they say. Instead, the planets may have formed further in or out and then migrated through the gassy disc to their current positions.