Super-Earths in two flavours
Astronomers suggest the super-Earths and sub-Neptunes might fall into two different density classes
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 "yes."
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