Astronomers announced recently
that they have discovered a new type of planet — a rocky world weighing 17
times as much as Earth. Theorists believed such a world couldn’t form because
anything so hefty would grab hydrogen gas as it grew and become a Jupiter-like
gas giant. This planet, though, is all solids and much bigger than previously
discovered “super-Earths,” making it a “mega-Earth.”
“We were very surprised
when we realized what we had found,” says astronomer Xavier Dumusque of the Harvard-Smithsonian
Center for Astrophysics (CfA), who led the data analysis and made the
discovery.
“This is the Godzilla
of Earths!” adds CfA researcher Dimitar Sasselov, director of the Harvard
Origins of Life Initiative. “But unlike the movie monster, Kepler-10c has
positive implications for life.”
The team’s finding was
presented today in a press conference at a meeting of the American Astronomical
Society (AAS).
The newfound
mega-Earth, Kepler-10c, circles a sunlike star once every 45 days. It is
located about 560 light-years from Earth in the constellation Draco. The system
also hosts a 3-Earth-mass “lava world,” Kepler-10b, in a remarkably fast,
20-hour orbit.
Kepler-10c was
originally spotted by NASA’s Kepler spacecraft. Kepler finds planets using the
transit method, looking for a star that dims when a planet passes in front of
it. By measuring the amount of dimming, astronomers can calculate the planet’s
physical size or diameter. However, Kepler can’t tell whether a planet is rocky
or gassy.
Kepler-10c was known to
have a diameter of about 18,000 miles, 2.3 times as large as Earth. This
suggested it fell into a category of planets known as mini-Neptunes, which have
thick, gaseous envelopes.
The team used the
HARPS-North instrument on the Telescopio Nazionale Galileo (TNG) in the Canary
Islands to measure the mass of Kepler-10c. They found that it weighed 17 times
as much as Earth — far more than expected. This showed that Kepler-10c must
have a dense composition of rocks and other solids.
“Kepler-10c didn’t lose
its atmosphere over time. It’s massive enough to have held onto one if it ever
had it,” explains Dumusque. “It must have formed the way we see it now.”
Planet formation
theories have a difficult time explaining how such a large, rocky world could
develop. However, a new observational study suggests that it is not alone.
Also presenting at AAS,
CfA astronomer Lars A. Buchhave found a correlation between the period of a
planet (how long it takes to orbit its star) and the size at which a planet
transitions from rocky to gaseous. This suggests that more mega-Earths will be
found as planet hunters extend their data to longer-period orbits.
The discovery that
Kepler-10c is a mega-Earth also has profound implications for the history of
the universe and the possibility of life. The Kepler-10 system is about 11
billion years old, which means it formed less than 3 billion years after the
Big Bang.
The early universe
contained only hydrogen and helium. Heavier elements needed to make rocky
planets, like silicon and iron, had to be created in the first generations of
stars. When those stars exploded, they scattered these crucial ingredients
through space, which then could be incorporated into later generations of stars
and planets.
This process should
have taken billions of years. However, Kepler-10c shows that the universe was
able to form such huge rocks even during the time when heavy elements were
scarce.
“Finding Kepler-10c
tells us that rocky planets could form much earlier than we thought. And if you
can make rocks, you can make life,” says Sasselov.
This research implies
that astronomers shouldn’t rule out old stars when they search for Earth-like
planets. And if old stars can host rocky Earths too, then we have a better
chance of locating potentially habitable worlds in our cosmic neighborhood.
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