Now that known exoplanets have become almost as numerous as fireflies on
a midsummer’s eve, two top planet-finding missions are starting to
disagree over the abundance of low-mass planets that are heavier than
Earth but smaller than Neptune.
The Swiss-led HARPS mission suggests that between 30 and 50 percent of
sunlike stars in the solar neighborhood host super-Earths and sub-
Neptunes. Meanwhile, NASA’s Kepler mission is finding that these planets
circle roughly 15 percent of the stars in its far-flung field of view.
That discrepancy is of great interest to astronomers, because the number
of planets in the weight class just above Earth hints at how many bodies
of terrestrial proportions are likely to be discovered.
But there may not be a discrepancy at all.
“We know the Geneva team does a good job observing, and they have a good
technique. And we know the Kepler telescope is working beautifully. So we
wanted to see if there was a plausible, believable way in which you could
have the difference between those two surveys,” says Greg Laughlin, an
astronomer at the University of California, Santa Cruz. In a paper posted
online August 30 at arXiv.org, he and U.C. Santa Cruz graduate student
Angie Wolfgang propose that there are two kinds of low-mass planets out
there, one of which is more amenable to discovery by HARPS.
Laughlin and Wolfgang created a simulation based on the HARPS data. In
it, they created a population of planets between one and 17 Earth-masses
around the more than 100,000 stars being monitored by Kepler. Giving
those virtual planets varying characteristics and orbital periods between
one and 50 days, the researchers then asked whether simulated planet
populations could reproduce the Kepler observations. The answer was
When peppered with two distinct kinds of planets in the size range — one
rocky and dense, the other gaseous and fluffy — the simulation mimics
Kepler’s data. “You don’t really have to force things into some strange
configuration. You can explain the results of those surveys if you have
these two distinct populations,” Laughlin says.
HARPS searches for distant planets by looking for the telltale signs of
an orbiting planet tugging on its host star in what’s called a radial
velocity survey. Surveys using the transit method, like Kepler, monitor a
star field for blips in brightness caused by a planet briefly blocking
some of the star’s light. These different methods can bias the types of
planets detected — denser planets tug more on their stars, and bloated
planets with bigger radii block more light.
“Nature can be quite a bit more clever than our simple models are,
especially in the realm of exoplanets. We always find that things are
much more interesting and complicated than we imagine,” says Caltech
planetary astronomer John Johnson.
Ideally, densities for detected planets could be found by peering at the
exo-worlds using both methods — but in practice that’s often tricky.
Two super-Earths that have been detected using both methods — CoRoT-7b
and GJ 1214b — have similar masses but different densities. Both are
examples that fit well with Wolfgang and Laughlin’s ideas, says Andrew
Youdin of the Harvard-Smithsonian Center for Astrophysics in Cambridge,
There are other explanations for the different results, though they’re
less probable. Johnson points to the possibility that the surveys are
peering at different stellar populations, but says that’s far-fetched.
Either way, there are lots and lots of planets out there, and some might
be quite familiar, Johnson says. “Kepler’s been telling us that the local
universe is teeming with Earths. They’re just all over the place. We need
to go out and find them.”