Distant Uranus-Sized Exoplanet Discovered With Gravitational Microlensing

Nasa's Hubble Space Telescope and the W M Keck Observatory in Hawaii made independent confirmations of the exoplanet orbiting far from its central star.

Microlensing technique can find distant and colder planets in long-period orbits that other methods cannot detect. Microlensing occurs when a foreground star amplifies the light of a background star that momentarily aligns with it. If the foreground star has planets, then the planets may also amplify the light of the background star, but for a much shorter period of time than their host star. The exact timing and amount of light amplification can give clues to the nature of the foreground star and its accompanying planets.

The system, catalogued as OGLE-2005-BLG-169, was discovered in 2005 by the Optical Gravitational Lensing Experiment (OGLE), the Microlensing Follow-Up Network (MicroFUN), and members of the Microlensing Observations in Astrophysics (MOA) collaborations.

Without conclusively identifying and characterising the foreground star, however, astronomers have had a difficult time determining the properties of the accompanying planet. Using Hubble and the Keck Observatory, two teams of astronomers have now found that the system consists of a Uranus-sized planet orbiting about 370 million miles from its parent star, slightly less than the distance between Jupiter and the Sun. The host star is about 70 percent as massive as our Sun.

"These chance alignments are rare, occurring only about once every 1 million years for a given planet, so it was thought that a very long wait would be required before the planetary microlensing signal could be confirmed," said David Bennett of the University of Notre Dame, Indiana, the lead of the team that analysed the Hubble data.

"Fortunately, the planetary signal predicts how fast the apparent positions of the background star and planetary host star will separate, and our observations have confirmed this prediction. The Hubble and Keck Observatory data, therefore, provide the first confirmation of a planetary microlensing signal," he said.

The planet in the OGLE system is probably an example of a "failed-Jupiter" planet, an object that begins to form a Jupiter-like core of rock and ice weighing around 10 Earth masses, but it does not grow fast enough to accrete a significant mass of hydrogen and helium, researchers said. So it ends up with a mass more than 20 times smaller than that of Jupiter.

"Failed-Jupiter planets, like OGLE-2005-BLG-169Lb, are predicted to be more common than Jupiters, especially around stars less massive than the Sun, according to the preferred theory of planet formation. So this type of planet is thought to be quite common," Bennett said.