Astronomers have found thousands
of planets orbiting distant stars, and the discoveries keep coming.
Yet many techniques detect only the planets closest to their host stars,
and within a few hundred light-years of Earth, leaving us to wonder what worlds
we're missing. A technique called microlensing promises to
clarify the picture. A recent analysis of six years of data
from the MOA-II ground-based survey concludes that exoplanets similar
in mass and, probably, composition, to Neptune are likely the most common
worlds in the outer reaches of planetary systems.
When a star passes directly between us and a more distant star,
its gravity can act like a lens, magnifying the background star's
brightness significantly for a few weeks. If the lensing star hosts
a planet, the planets gravity can produce a noticeable change in brightness over
a hours or days. This spike signals not only the planet's
presence, but tells us its mass and distance from the star.
Each method of finding exoplanets
has different strengths. Radial velocity measurements reveal planets by
detecting how they cause the star to move. Transit measurements reveal dips in
starlight caused by planets passing in front of their stars.
Both work best for massive planets in close orbits, and for stars up to hundreds of
lgiht-years away. Microlensing opens a
planetary window onto a larger part of the galaxy, reaching thousands of light-years.
And because microlensing is more sensitive to smaller planets farther from their stars,
it can reveal new planetary populations.
In the MOA-II study, researchers discovered that planets beyond a certain
distance from their star tend to be roughly 20 Earth masses, or
about the same as Neptune. That distance is what astronomers call
the "snow line," where water would be frozen during the formation of a
planetary system. For our system, that location is roughly
2.7 times farther from the sun than Earth. Beyond the snow line,
where there is more solid material to coagulate and initiate the planet formation
process, planetary formation is thought to be most efficient.
In fact, worlds formed in this frozen hinterland
may play an important role in making habitable planets closer to their star.
The gravity of planets beyond the snow line can help send water-rich
asteroids inward, where they can deliver water to young rocky worlds.
WFIRST, an upcoming NASA mission,
which combines high-resolution with a huge field of view, will watch for
microlensing events toward the central part of our galaxy, the Milky Way.
It will expand on the exoplanet survey started by NASA's Kepler mission,
and should reveal exoplanets down to Mars mass in orbits around their
stars as close as Earth's to more distant than Neptune's.
When combined with Kepler's discoveries, WFIRST will give us a complete
picture of exoplanetary systems. Stay tuned.
[Music]
[Beeping]