Odds of Finding Earth-Size Exoplanets Are 1-in-4

Nearly one in four sun-like stars should host an Earth-mass planet,
according to a new census.

The finding is the first quantitative measurement of the frequency of
planets of various masses in the galaxy.

“It’s a landmark paper,” said exoplanet expert Josh Winn of MIT, who
was not involved in the new study. “There’s been all this talk, that
low-mass planets like the Earth are very common. But this is the first
time it’s been documented.”

The study, published in the Oct. 29 Science, also found plenty of
planets in a mass range that astronomers expected to be empty, which
may prompt an overhaul of planet-formation models.

Using the Keck Observatory in Hawaii, astronomer Andrew Howard of the
University of California, Berkeley and colleagues watched 166 sun-like
stars for the telltale wobbles induced by a planet’s pull.

Unlike previous surveys, Howard and colleagues were just as interested
in stars that lack planets as stars that host them. To avoid biasing
the study toward planet-bearing stars, the team selected the nearest
and brightest stars in the 120,000-star Hipparcos Catalog.

“It’s hard to write a telescope proposal that says, ‘We want to look
at these stars because we think they don’t have planets,’” Howard
said. “But that’s what we had to do.”

Over the course of five years, the team observed each star at least 20
times in search of planets that have masses between 3 and 1,000 times
the Earth’s mass and that orbit close to their stars.

They found 33 planets around 22 of the stars, some of which had
already been discovered and reported by other groups, and 12 candidate
planets that still need to be confirmed. Because some stars were
observed more often than others, the team included a “missing-planet
correction” to account statistically for planets that would probably
show up with more observations.

None of these planets was actually the same mass as Earth.
Astronomers’ instruments aren’t yet sensitive enough to detect such
small worlds.

“But what we can do is extrapolate,” Howard said. “It involves a
little bit of speculation, but we’re comfortable with that
uncertainty.”

In general, small planets turned out to be much more common than large
ones. The researchers extended that trend down to planets about half
Earth’s mass.

They found that about 23 percent (give or take about 10 percent) of
sun-like stars should have a planet between half and twice the Earth’s
mass orbiting very close in, about a quarter of the distance from the
Earth to the sun. That distance would make the planets far too warm
for liquid water. But because planets tend to be more abundant farther
from their stars, Howard thinks there should be even more Earth-mass
planets in cooler orbits where liquid water is stable.

“I wouldn’t be surprised if the true number [of stars hosting
Earth-mass planets] is one in two, or one in eight — but I’m almost
sure it’s not one in 100,” he said. “That’s a really big improvement
on our knowledge.”

Surprisingly, the observations also showed a lot of planets between 5
and 30 times Earth’s mass, a range that theoretical models of planet
formation predicted should be so empty it earned the name, “the planet
desert.”

“We showed that the desert is in fact closer to a tropical rain
forest,” Howard said.

The new numbers are a windfall to researches like Winn, who are
involved in designing the next generation of planet-hunting
telescopes.

“It sets our expectations much more clearly than they were last week,”
he said. “We were just guessing, to see how to design the instrument.
Now we have some much more solid numbers to put in.”

Exoplanet expert Sara Seager of MIT, who was not involved in the new
study, noted that this is one of the first exoplanet papers that
doesn’t focus on just one planet or one system of planets.

“Exoplanet [research] is moving from single-planet characterization to
statistics,” she said. Thanks to the statistical treatment, Seager is
more willing to accept uncertainties in Howard’s analysis.

“You might not have confirmation of every individual planet, but
that’s OK,” she said. “That shouldn’t stop you from making general
statements.”

The study also prepares the way for the deluge of planets that should
come from the Kepler Space Telescope in the next few years, she says.
“We want the world to accept things from Kepler, but they have to be
primed to believe that statistics are OK.”

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