Newfound Alien Planet Is Best Place Yet to Search for Life

[Garrison L. Hilliard]

A newly discovered planet around a distant star may jump to the top of
the list of places where scientists should go looking for alien life.

The alien world known as LHS 1140b is rocky, like Earth. It is only 40
light-years away from our solar system (essentially, down-the-street
in cosmic terms), and sits in the so-called habitable zone of its
parent star, which means liquid water could potentially exist on the
planet's surface. Several other planets also meet those criteria, but
few of them are as prime for study as LHC 1140b according to the
scientists who discovered it, because the type of star the planet
orbits and the planet's orientation to Earth make it ripe for
investigations into whether it’s the kind of place where life could
thrive.

"This is the most exciting exoplanet I've seen in the past decade,"
Jason Dittmann, a postdoctoral fellow at the Harvard-Smithsonian
Center for Astrophysics (CfA) and lead author on the paper describing
the discovery, said in a statement from CfA. "We could hardly hope for
a better target to perform one of the biggest quests in science —
searching for evidence of life beyond Earth." [10 Exoplanets That
Might Be Perfect to Support Life]


Alien atmosphere

Thousands of exoplanets have been discovered orbiting stars other than
the sun in the last 20 years. Many of those planets meet some of the
basic requirements for hosting life as we know it — they're rocky like
Earth (rather than gaseous, like Saturn or Jupiter) and they sit in
the habitable zone of their parent star.

LHS 1140b meets those initial requirements. Through multiple
observations, Dittmann and colleagues determined that the planet
receives about 0.46 times as much light from its parent star as Earth
receives from the sun. The planet is about 1.4 times the diameter of
Earth and 6.6 times its mass, which makes it a so-called super-Earth
and suggests it is also rocky. [How Habitable Zones for Alien Planets
and Stars Work (Infographic)]

The next step scientists are taking to find out if exoplanets like LHS
1140b are habitable (or even inhabited) is to examine their
atmospheres. An atmosphere could provide life-forms with a necessary
ingredient for life (such as oxygen or carbon dioxide on Earth), and
could also bear signs that life exists there (most of the methane on
Earth, for example, is produced by biological organisms). Scientists
are working on understanding what the atmosphere of an exoplanet can
reveal about the likelihood that it hosts life, or could.

Dittmann said he and his colleagues think LHS 1140b is a great
candidate for follow-up atmospheric studies for multiple reasons.

This alien world was initially discovered using the transit method, in
which scientists look at the light from a star and try to measure
subtle dips in its brightness that could be caused by a planet passing
in front of (transiting) the star. In some cases, telescopes can
capture the sliver of sunlight that passes through the planet's
atmosphere, and that sunlight reveals information about the chemical
composition of the planet's atmosphere. Many other potentially
habitable Earth-like planets ? such as Proxima b, the closest
exoplanet to our solar system that lies only 4.2 light-years away ? do
not transit their parent star as seen from Earth and therefore their
atmospheres can't be studied in this way.

The team's precise measurement of LHS 1140b's density will also be
important to understanding its atmosphere, Dittmann told Space.com.

"What's great about having a density ahead of an atmospheric study is
that this density tells you how tightly the planet holds on to its
atmosphere (the atmospheric scale height)," Dittmann told Space.com in
an email. Using the transit method, scientists are trying to collect
starlight shining through a planet's atmosphere; a thicker atmosphere
means more light passes through it, making it easier for scientists to
detect the signals from various chemical elements present in that
atmosphere. A planet with higher density also has stronger gravity,
which further compresses the atmosphere and reduces the size of the
signals scientists can detect.

An artist's illustration of the view from exoplanet LHS 1140b, which
orbits an M dwarf star about one-fifth the size of the sun. The planet
could be a prime target for follow-up studies in the search for life
elsewhere in the universe.
An artist's illustration of the view from exoplanet LHS 1140b, which
orbits an M dwarf star about one-fifth the size of the sun. The planet
could be a prime target for follow-up studies in the search for life
elsewhere in the universe.
Credit: M. Weiss/CfA
But clouds can also reduce the size of the signal by simply blocking
the light coming through the atmosphere, Dittmann said.

"Since these two things have similar effects, you can't disentangle
the two," he said. "Here, having a mass measurement is super helpful
because then you already know the effect of the mass of the planet,
and anything 'extra' can be due to clouds."

Dittmann and colleagues made that precise density measurement of LHS
1140b through a different method known as the radial velocity
technique, in which scientists look for the way an exoplanet tugs on
its parent star. Precise measurements of the mass and density of an
exoplanets are also not entirely rare, but can be difficult to
determine in some systems, as is the case for the recently discovered
crop of seven exoplanets orbiting a single star in the TRAPPIST-1
system, which is about 39 light-years from Earth.

"Only one of those worlds has had its density measured accurately,
showing that it isn’t rocky," according to the statement from CfA.
"Therefore, some or all of the others also might not be rocky."

A cool star

Another reason Dittmann and his colleagues think LHS 1140b is a good
follow-up in the search for life is because of the kind of star it
orbits, even though that star is very different from the sun.

The star LHS 1140 is an M dwarf star (also known as a red dwarf). It
is only one-fifth the size of Earth's sun and significantly cooler.
But it's extremely difficult to study exoplanets that orbit close to a
bright star, because the light from the star drowns out the light from
the planet. Around a cooler, dimmer star, that problem is slightly
alleviated. In addition, M dwarfs are the most common type of star in
the galaxy, which has led some scientists to push for planet searches
that target red dwarf stars.

But these dim red stars can also be violent in their early lives,
pelting infant planets with harsh ultraviolet radiation and X-rays,
potentially evaporating liquid water or snuffing out early forms of
life. The star LHS 1140 is a relatively quiet red dwarf, according to
the new paper. By comparison, the star at the center of the TRAPPIST-1
system produces more frequent bursts of harsh radiation and has been
found to radiate strongly in X-ray wavelengths, Dittmann said. For a
planet to sit in the habitable zone around a dim red star, it must
orbit much closer to the star than Earth orbits the sun, which can
make planets even more susceptible to the harsh effects of the star's
radiation.

Dittmann said the team has been approved to use the Hubble Space
Telescope to get a better look at the star and see how bright it is in
those ultraviolet and X-ray wavelengths. ("We expect it to be very
dim, but it's always good to check!" he said.)

The team also plans to use Hubble to start gathering data about LHS
1140b's atmosphere, in anticipation of being able to study it with
larger telescopes, such as the James Webb Space Telescope, set to
launch in 2018, and the Giant Magellan Telescope and the Thirty Meter
Telescope, set to come online in the 2020s.

Scientists may very well not find life on LHS 1140b, but this perfect
storm of characteristics makes it the perfect subject to teach
scientists about how planets around M dwarfs evolve.

"M dwarfs are the most common type of star in the galaxy, and the
discovery of LHS 1140b provides us with an excellent opportunity to
learn more about whether planets orbiting these stars are habitable,"
said Victoria Meadows, a professor of astronomy at the University of
Washington, who was not involved in the research. "If planets like LHS
1140b orbiting M dwarfs can be habitable, then it will increase the
potential prevalence of life throughout the galaxy."

Follow Calla Cofield @callacofield. Follow us @Spacedotcom, Facebook
and Google+. Original article on Space.com.


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