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Astronomers have found thousands

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of planets orbiting distant stars, and the discoveries keep coming.

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Yet many techniques detect only the planets closest to their host stars,

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and within a few hundred light-years of Earth, leaving us to wonder what worlds

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we're missing. A technique called microlensing promises to

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clarify the picture. A recent analysis of six years of data

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from the MOA-II ground-based survey concludes that exoplanets similar

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in mass and, probably, composition, to Neptune are likely the most common

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worlds in the outer reaches of planetary systems.

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When a star passes directly between us and a more distant star,

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its gravity can act like a lens, magnifying the background star's

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brightness significantly for a few weeks. If the lensing star hosts

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a planet, the planets gravity can produce a noticeable change in brightness over

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a hours or days. This spike signals not only the planet's

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presence, but tells us its mass and distance from the star.

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Each method of finding exoplanets

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has different strengths. Radial velocity measurements reveal planets by

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detecting how they cause the star to move. Transit measurements reveal dips in

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starlight caused by planets passing in front of their stars.

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Both work best for massive planets in close orbits, and for stars up to hundreds of

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lgiht-years away. Microlensing opens a

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planetary window onto a larger part of the galaxy, reaching thousands of light-years.

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And because microlensing is more sensitive to smaller planets farther from their stars,

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it can reveal new planetary populations.

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In the MOA-II study, researchers discovered that planets beyond a certain

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distance from their star tend to be roughly 20 Earth masses, or

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about the same as Neptune. That distance is what astronomers call

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the "snow line," where water would be frozen during the formation of a

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planetary system. For our system, that location is roughly

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2.7 times farther from the sun than Earth. Beyond the snow line.

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where there is more solid material to coagulate and initiate the planet formation

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process, planetary formation is thought to be most efficient.

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In fact, worlds formed in this frozen hinterland

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may play an important role in making habitable planets closer to their star.

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The gravity of planets beyond the snow line can help send water-rich

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asteroids inward, where they can deliver water to young rocky worlds.

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WFIRST, an upcoming NASA mission,

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which combines high-resolution with a huge field of view, will watch for

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microlensing events toward the central part of our galaxy, the Milky Way.

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It will expand on the exoplanet survey started by NASA's Kepler mission,

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and should reveal exoplanets down to Mars mass in orbits around their

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stars as close as Earth's to more distant than Neptune's.

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When combined with Kepler's discoveries, WFIRST will give us a complete

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picture of exoplanetary systems. Stay tuned.

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