Abstract
We detect a cold, terrestrial planet in a binary-star system using
gravitational microlensing. The planet has low mass (2 Earth masses) and lies
projected at $a_\perp,ph$ ~ 0.8 astronomical units (AU) from its host star,
similar to the Earth-Sun distance. However, the planet temperature is much
lower, T<60 Kelvin, because the host star is only 0.10--0.15 solar masses and
therefore more than 400 times less luminous than the Sun. The host is itself
orbiting a slightly more massive companion with projected separation
$a_\perp,ch=$10--15 AU. Straightforward modification of current microlensing
search strategies could increase their sensitivity to planets in binary
systems. With more detections, such binary-star/planetary systems could place
constraints on models of planet formation and evolution. This detection is
consistent with such systems being very common.
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