Distant Uranus-sized planet discovered

An ongoing hunt to find exoplanets orbiting far from their own star has made a welcome discovery.

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Diagram explaining how the microlensing technique was used to discover the exoplanet around OGL-2005-BLG-169. Image Credit: Hubble/STScl

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NASA’s Hubble Space Telescope and the W.M. Keck Observatory in Hawaii have both independently confirmed the discovery of an exoplanet orbiting far from its central star.

The discovery was made as part of efforts to find distant planets orbiting as far from their parent star as Jupiter and Saturn are from the Sun.

The gas giant planet was revealed around the OGLE-2005-BLG-169 system and its discovery has opened up a new section of space in the hunt for exoplanets.

Most distant planets catalogued so far are close to their host stars, due to the methods used to find them, such as the Radial Velocity Method, which determines the existence of planets through the gravitational force they exert on their star, and the transit method, which looks for dips in the brightness of a star as a planet passes in front of it.

These techniques are mostly effective in locating planets with short-period orbits.

This latest exoplanet discovery used a technique called microlensing, which is able to locate planets farther away in longer-period orbits.

The technique involves observing the point at which a foreground star amplifies the light of a background star that aligns with it.

If the foreground star has planets, these can also amplify the light of the background star. Noting the timing and light amplification reveals clues as to the nature of the foreground star and its orbiting planets.

In this case, however, astronomers initially struggled to determine the properties of the accompanying planet.

Follow-up observations with Hubble and Keck found that the system consists of a Uranus-sized planet orbiting about 600 million kilometres from its parent star, which is just shy of the distance between Jupiter and the Sun. This particular planet’s host star, however, is about 70 per cent as big as our own.

“These chance alignments are rare, occurring only about once every one 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,” says David Bennett, who led 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.”

“It is remarkable that we can detect planets orbiting unseen stars, but we’d really like to know something about the stars that these planets orbit,” says Virginie Batista, leader of the Keck Observatory analysis.

“The Keck and Hubble telescopes allow us to detect these faint planetary host stars and determine their properties.”

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The OGLE-2005-BLG-169 system 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.