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