To us, the solar system seems normal. But new studies of the planets
sprinkled around other stars suggest that our corner of the galaxy is
actually a pretty weird place.
Scientists have discovered that a
large proportion of the recently discovered planets outside our own
solar system are nothing like the familiar planets orbiting our sun.
Instead, many are "mini-Neptunes": roughly the size of Earth but, unlike
Earth, composed of a thick layer of gases around a solid core, which is
more like the composition of the planet Neptune. That implies that the
recipe for making planets in other solar systems is far different than
the process that led to Earth's formation.
"dominate the inventory" of the 3,000-plus planets found by NASA's
planet-hunting spacecraft Kepler, said University of California-Berkeley
astronomer Geoff Marcy on Monday at an American Astronomical Society
meeting just outside Washington, D.C. "These are planets we never
expected based on our own solar system."
Marcy and his team examined dozens of "exoplanets" - planets that
circle a star other than the sun - spotted by Kepler before it went bust
last year. With the keen-eyed Keck telescope in Hawaii, Marcy's team
looked for tiny perturbations that planets create in their stars. The
bigger the perturbation, the more massive the planet.
researchers found that exoplanets fall into two groups. Planets that are
roughly twice the size of Earth or smaller have a rocky core plus some
water, like Earth. But planets two to four times the size of Earth have a
rocky core but are also built of lots of gas, like Neptune.
findings of Marcy's team were validated by scientists who turned to a
different method of sizing up planets. Northwestern University's Yoram
Lithwick said Monday that his team measured the heft of a selection of
exoplanets by examining how two planets orbiting the same star change
each other's journey around their star. The bigger the change, the more
massive the planet.
Lithwick and his colleagues examined some 60 exoplanets smaller than
Neptune but bigger than Earth and found that roughly one in three are
small, fluffy bodies containing high amounts of gas. That composition
means that unlike Earth, which formed relatively late in the solar
system's history, these fluffball planets formed "much earlier. So these
planets must be dramatically different from the Earth, contrary to what
we expected," Lithwick says.
Yet another lightweight planet
called KOI-314c is almost exactly the mass of Earth but much larger
across, meaning it, too, must have a thick atmosphere of gas, David
Kipping of the Harvard-Smithsonian Center for Astrophysics said at the
Despite its Earth-like measurements, "it's nothing
like Earth," Kipping said. Until now, he would've assumed an exoplanet
with Earth's mass would also have the same basic ingredient as Earth,
namely rock. "You can't just draw a line in the sand ... and say,
everything below this point is rocky, everything above that point is
So why don't we have a mini-Neptune in our own solar system?
20% to 30% of stars have "these crazy planets," Lithwick said. "You
could say that all these planets Kepler has found are strange. But
another way of looking at it is it's really the solar system that may be