Researchers advocate statistical approach to search for Earth-like planets

A team of astronomers at the University of Chicago and
Grinnell College seeks to change the way scientists approach
the search for Earth-like planets orbiting stars other than
the sun. They favor taking a statistical comparative approach
in seeking habitable planets and life beyond the solar
system.

“The nature of proof should not be: ‘Can we point at a planet
and say, yes or no, that’s the planet hosting alien life,” said
Jacob Bean, associate professor of astronomy and astrophysics
at UChicago. “It’s a statisical exercise. What can we say for
an ensemble of planets about the frequency of the existence of
habitable environments, or the frequency of the existence of
life on those planets?”

The standard approach of researching exoplanets, or planets
that orbit distant stars, has entailed studying small numbers
of objects to determine if they have the right gases in the
appropriate quantities and ratios to indicate the existence of
life. But in a recent paper with co-authors Dorian Abbot and
Eliza Kempton in the Astrophysical Journal Letters, Bean
describes the need “to think about the techniques and
approaches of astronomy in this game—not as planetary
scientists studying exoplanets.”

“Nature has provided us with huge numbers of planetary
systems,” said Kempton, an assistant professor of physics at
Grinnell College in Iowa. “If we survey a large number of
planets with less detailed measurements, we can still get a
statistical sense for how prevalent are in our galaxy.
This would give us a basis for future, more detailed surveys.”

Kempton and Bean attest to the challenges of making detailed
observations of a potentially Earth-like planet. Together they
have previously studied the super-Earth known as GJ 1214b, an
exoplanet with a mass greater than Earth’s but less than gas
giants such as Neptune and Uranus. GJ 1214b turned out to be
quite cloudy, which prevented them from determining the
composition of its atmosphere.

“A large statistical study will allow us to look at many
planets,” Kempton said. “If any single object proves to be
particularly challenging to observe, like GJ 1214b, that won’t
be a major loss to the observing program on the whole.”

Kepler observatory a game-changer

The inspiration for the paper stemmed from Bean’s membership on
the Science and Technology Definition Team that is assessing
the potential for a new space telescope, NASA’s proposed Large
UV/Optical/Infrared Survey (LUVOIR).

One of LUVOIR’s scientific priorities is the search for
Earth-like planets. During one team meeting, Bean and his
colleagues listed all the properties of a potentially habitable
exoplanet that they need to measure and how they would go about
obtaining the data. Given the current state of technology, Bean
concluded that it’s unlikely scientists will be able to confirm
an individual exoplanet as suitable for life or whether life is
actually there.

Nevertheless, astronomers have gathered an impressive haul of
exoplanetary data from NASA’s Kepler space observatory, which
has operated since 2009.

“Kepler completely changed the game,” Bean said. “Instead of
talking about a few planets or a few tens of planets, all of a
sudden we had a few thousand planet candidates. They were
planet candidates because Kepler couldn’t definitely prove that
the signal it was seeing was due to planets.”

The standard approach has been to take additional observations
for each candidate to rule out possible false positive
scenarios, or to detect the planet with a second technique.

“That’s very slow going. One planet at a time, a lot of
different observations,” Bean noted. But an alternative is to
make statistical calculations for the probability of false
positives among these thousands of exoplanet candidates. That
new approach led directly to a good understanding of the
frequency of exoplanets of different sizes. For example,
scientists now can say that the frequency of super-Earth-type
planets is 15 percent, plus or minus 5 percent.

Role of spectroscopy

Spectroscopic studies play a key role in characterizing
exoplanets. This involves determining the composition of a
planetary atmosphere by measuring its spectra, the distinctive
radiation that gases absorb at their own particular
wavelengths. Bean and his co-authors suggest focusing on what
can be learned from measuring the spectra of a large ensemble
of terrestrial exoplanets.

Spectroscopy may, for example, help exoplanetary researchers
verify a phenomenon called the silicate weathering feedback,
which acts as a planetary thermostat. Through silicate
weathering, the amount of varies according
to geologic processes. Volcanoes emit carbon dioxide into the
atmosphere, but rain and chemical reactions that occur in rocks
and sediments also remove the gas from the atmosphere.

Rising temperatures would put more water vapor into the
atmosphere, which then rains out, increasing the amount of
dissolved carbon dioxide that chemically interacts with the
rocks. This loss of carbon dioxide from the atmosphere has a
cooling effect. But as a planet begins to cool, rock weathering
slows and the amount of carbon dioxide gradually builds from
its volcanic sources, which causes rising temperatures.

Global-scale observations suggest that Earth has experienced
silicate-weathering feedback. But attempts to verify that the
process is operating today on the scale of individual river
basins has proven difficult.

“The results are very noisy. There’s no clear signal,” Abbot
said. “It would be great to have another independent
confirmation from exoplanets.”

All three co-authors are interested in fleshing out the details
of experiments they proposed in their paper. Abbot plans to
calculate how much carbon dioxide would be necessary to keep a
planet habitable at a range of stellar radiation intensities
while changing various planetary parameters. He also will
assess how well a future instrument would be able to measure
the gas.

“Then we will put this together to see how many planets we
would need to observe to detect the trend indicating a
silicate-weathering feedback,” Abbot explained.

Bean and Kempton, meanwhile, are interested in detailing what a
statistical census of biologically significant gases such as
oxygen, and ozone could reveal
about planetary habitability.

“I’d like to get a better understanding of how some of the
next-generation telescopes will be able to distinguish
statistical trends that indicate habitable—or
inhabited—planets,” Kempton said.

Explore further:

Astrophysicists identify composition of earth-size planets in
TRAPPIST-1 system

More information: Jacob L. Bean et al. A Statistical
Comparative Planetology Approach to the Hunt for Habitable
Exoplanets and Life Beyond the Solar System, The
Astrophysical Journal
(2017). DOI: 10.3847/2041-8213/aa738a