NASA and NSF-Funded Research Finds First Potentially Habitable Exoplanet

WASHINGTON -- A team of planet hunters from the University of California (UC) Santa Cruz, and the Carnegie Institution of Washington has announced the discovery of a planet with three times the mass of Earth orbiting a nearby star at a distance that places it squarely in the middle of the star's "habitable zone."

This discovery was the result of more than a decade of observations using the W. M. Keck Observatory in Hawaii, one of the world's largest optical telescopes. The research, sponsored by NASA and the
National Science Foundation, placed the planet in an area where
liquid water could exist on the planet's surface. If confirmed, this
would be the most Earth-like exoplanet yet discovered and the first
strong case for a potentially habitable one.

To astronomers, a "potentially habitable" planet is one that could
sustain life, not necessarily one where humans would thrive.
Habitability depends on many factors, but having liquid water and an
atmosphere are among the most important.

The new findings are based on 11 years of observations of the nearby
red dwarf star Gliese 581using the HIRES spectrometer on the Keck I
Telescope. The spectrometer allows precise measurements of a star's
radial velocity (its motion along the line of sight from Earth),
which can reveal the presence of planets. The gravitational tug of an
orbiting planet causes periodic changes in the radial velocity of the
host star. Multiple planets induce complex wobbles in the star's
motion, and astronomers use sophisticated analyses to detect planets
and determine their orbits and masses.

"Keck's long-term observations of the wobble of nearby stars enabled
the detection of this multi-planetary system," said Mario R. Perez,
Keck program scientist at NASA Headquarters in Washington. "Keck is
once again proving itself an amazing tool for scientific research."
Steven Vogt, professor of astronomy and astrophysics at UC Santa Cruz,
and Paul Butler of the Carnegie Institution lead the Lick-Carnegie
Exoplanet Survey. The team's new findings are reported in a paper
published in the Astrophysical Journal and posted online at:


"Our findings offer a very compelling case for a potentially habitable
planet," said Vogt. "The fact that we were able to detect this planet
so quickly and so nearby tells us that planets like this must be really common."

The paper reports the discovery of two new planets around Gliese 581.
This brings the total number of known planets around this star to
six, the most yet discovered in a planetary system outside of our
own. Like our solar system, the planets around Gliese 581 have
nearly-circular orbits.

The new planet designated Gliese 581g has a mass three to four times
that of Earth and orbits its star in just under 37 days. Its mass
indicates that it is probably a rocky planet with a definite surface
and enough gravity to hold on to an atmosphere.
Gliese 581, located 20 light years away from Earth in the
constellation Libra, has two previously detected planets that lie at
the edges of the habitable zone, one on the hot side (planet c) and
one on the cold side (planet d). While some astronomers still think
planet d may be habitable if it has a thick atmosphere with a strong
greenhouse effect to warm it up, others are skeptical. The
newly-discovered planet g, however, lies right in the middle of the
habitable zone.

The planet is tidally locked to the star, meaning that one side is
always facing the star and basking in perpetual daylight, while the
side facing away from the star is in perpetual darkness. One effect
of this is to stabilize the planet's surface climates, according to
Vogt. The most habitable zone on the planet's surface would be the
line between shadow and light (known as the "terminator").

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Source: NASA

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