An ice giant that resembles our Solar System’s Uranus has been discovered about 25,000 light-years from Earth, by a multinational team of scientists led by Dr Radosław Poleski of Ohio State University and Warsaw University Observatory in Poland.
Composite image of Uranus. Image credit: Marcos van Dam / W. M. Keck Observatory.
The planet, labeled OGLE-2008-BLG-092LAb (092LAb for short), has a mass of about 4 times that of Uranus.
It is part of a binary star system in the constellation Sagittarius.
The first star in the system, 092LA, is about two thirds as massive as our Sun, and the second star, 092LB, is about one sixth as massive.
The planet orbits 092LA at 18 AU (astronomical units) – almost exactly the same distance as Uranus orbits the Sun.
Dr Poleski and his colleagues discovered this planetary system due to a phenomenon called gravitational microlensing – when the gravity of a star focuses the light from a more distant star and magnifies it like a lens. Very rarely, the signature of a planet orbiting the lens star appears within that magnified light signal.
In this case, there were two separate microlensing events, one in 2008 that revealed 092LA and suggested the presence of the planet, and one in 2010 that confirmed the presence of the planet and revealed the second star, 092LB.
“Only microlensing can detect these cold ice giants that, like Uranus and Neptune, are far away from their host stars. This discovery demonstrates that microlensing is capable of discovering planets in very wide orbits,” Dr Poleski said.
Both observations were done with the 1.3-m Warsaw Telescope at Las Campanas Observatory in Chile as part of the Optical Gravitational Lensing Experiment (OGLE).
“This discovery may help solve a mystery about the origins of the ice giants in our Solar System,” said Prof Andrew Gould of Ohio State University, who is the senior author of the paper published in the Astrophysical Journal (arXiv.org preprint).
“Nobody knows for sure why Uranus and Neptune are located on the outskirts of the Solar System, when our models suggest that they should have formed closer to the Sun. One idea is that they did form much closer, but were jostled around by Jupiter and Saturn and knocked farther out.”
He added: “maybe the existence of this Uranus-like planet is connected to interference from the second star; maybe you need some kind of jostling to make planets like Uranus and Neptune.”
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Radosław Poleski et al. 2014. Triple Microlens OGLE-2008-BLG-092L: Binary Stellar System with a Circumprimary Uranus-type Planet. ApJ 795, 42; doi: 10.1088/0004-637X/795/1/42
