By Ron Cowen
Astronomers who previously imaged three giant planets around a single star have once again hit the jackpot by finding a fourth large body in the star system. More massive than Jupiter, the newly discovered planet lies considerably closer to the nearby star HR 8799 than the other three planets, raising questions about how all four giants formed.
The planet’s discoverers think so at least. But some other researchers disagree, saying that the situation may not be all that difficult to explain with current planet formation theories.
The star HR 8799 and its planetary retinue lie just 130 light-years from Earth. Captured in a series of telescope images over 15 months, the newly found planet lies about 14.5 astronomical units from its sun. That puts the planet about midway between where Saturn and Uranus sit in the solar system and much closer than its three companions, which the team unveiled two years ago (SN: 12/6/08, p. 5).
The planets orbiting HR 8799, with masses estimated at between five and 10 times that of Jupiter, form a scaled-up version of the planets in the outer solar system, according to codiscoverer Christian Marois of the Herzberg Institute of Astrophysics in Victoria, Canada, and his colleagues. They describe their findings, based on observations with the Keck II telescope atop Hawaii’s Mauna Kea in 2009 and 2010, in an article posted on arXiv.org on November 23 (http://arxiv.org/abs/1011.4918).