Wednesday, August 27, 2014

A Uranus-Type Planet in a Binary Stellar System

A gravitational microlensing search by R. Poleski (2014) revealed the presence of a Uranus-type planet in orbit around a 0.6 solar mass star. The gravitational microlensing event is designated OGLE-2008-BLG-092, and the newfound planet is estimated to be ~3 times the mass of Uranus and it circles its host star at ~16 AU. For comparison, Uranus orbits the Sun at an average distance of 19 AU. This newfound planet is the first known exoplanet whose mass and orbit is similar to Uranus. The planet was detected when it and its host star fortuitously passed in front of a background star, and the gravitational field of the star-planet system magnified light from the background star.

Figure 1: Artist’s impression of a Uranus-type planet.

Planets in the Solar System can be classed into 3 groups: small rocky planets (Earth, Venus, etc), gas giants (Jupiter and Saturn) and ice-giants (Uranus and Neptune). At present, the leading methods of detecting planets around other stars (i.e. transit and radial velocity methods) have yet to turn up any extrasolar analogues of Uranus and Neptune. Such planets are far from their host stars and have orbital periods that exceed a human lifespan. As a result, both the transit and radial velocity methods have yet to turn up such planets since both methods greatly favour the detection of planets with short orbital periods. To detect extrasolar analogues of Uranus and Neptune using such methods would require exceedingly long observation timescales.

Although the technique of direct imagine can detect planets that orbit far from their host stars, this technique has so far been restricted to the detection of more massive and hotter planets that inhibit young planetary systems. These planets are very different from planets like Uranus and Neptune. At present, the only method that can detect extrasolar analogues of Uranus and Neptune seems to be gravitational microlensing as this method allows planets to be detected regardless of their orbital periods. In addition to the Uranus-type planet and its host star, the OGLE-2008-BLG-092 microlensing event also revealed the presence of a companion object in the system that is either a low mass star or a brown dwarf. In fact, the projected separation of the Uranus-type planet from its host star is only ~3 times smaller than that of the companion star (or brown dwarf).

Figure 2: Light curve of the OGLE-2008-BLG-092 microlensing event. The inset shows the planetary subevent. The presence of the companion star (or brown dwarf) is indicated by the 2010 subevent. R. Poleski (2014).

R. Poleski (2014), “Triple Microlens OGLE-2008-BLG-092L: Binary Stellar System with a Circumprimary Uranus-type Planet”, arXiv:1408.6223 [astro-ph.EP]