A new study finds
that we could detect oxygen in the atmosphere of a habitable planet
orbiting a white dwarf (as shown in this artist's illustration) much
more easily than for an Earth-like planet orbiting a Sun-like star. Here
the ghostly blue ring is a planetary nebula - hydrogen gas the star
ejected as it evolved from a red giant to a white dwarf. Credit: David A. Aguilar (CfA). High Resolution Image (jpg) - Low Resolution Image (jpg)
Cambridge, MA - Even dying stars could host planets with life - and if such life exists, we might be able to detect it within the next decade. This encouraging result comes from a new theoretical study of Earth-like planets orbiting white dwarf stars. Researchers found that we could detect oxygen in the atmosphere of a white dwarf's planet much more easily than for an Earth-like planet orbiting a Sun-like star.
"In the quest for extraterrestrial biological signatures, the first
stars we study should be white dwarfs," said Avi Loeb, theorist at the
Harvard-Smithsonian Center for Astrophysics (CfA) and director of the
Institute for Theory and Computation.
When a star like the Sun dies, it puffs off its outer layers, leaving
behind a hot core called a white dwarf. A typical white dwarf is about
the size of Earth. It slowly cools and fades over time, but it can
retain heat long enough to warm a nearby world for billions of years.
Since a white dwarf is much smaller and fainter than the Sun, a planet
would have to be much closer in to be habitable with liquid water on its
surface. A habitable planet would circle the white dwarf once every 10
hours at a distance of about a million miles.
Before a star becomes a white dwarf it swells into a red giant,
engulfing and destroying any nearby planets. Therefore, a planet would
have to arrive in the habitable zone after the star evolved into a white
dwarf. A planet could form from leftover dust and gas (making it a
second-generation world), or migrate inward from a larger distance.
If planets exist in the habitable zones of white dwarfs, we would need
to find them before we could study them. The abundance of heavy elements
on the surface of white dwarfs suggests that a significant fraction of
them have rocky planets. Loeb and his colleague Dan Maoz (Tel Aviv
University) estimate that a survey of the 500 closest white dwarfs could
spot one or more habitable Earths.
The best method for finding such planets is a transit search - looking
for a star that dims as an orbiting planet crosses in front of it. Since
a white dwarf is about the same size as Earth, an Earth-sized planet
would block a large fraction of its light and create an obvious signal.
More importantly, we can only study the atmospheres of transiting
planets. When the white dwarf's light shines through the ring of air
that surrounds the planet's silhouetted disk, the atmosphere absorbs
some starlight. This leaves chemical fingerprints showing whether that
air contains water vapor, or even signatures of life, such as oxygen.
Astronomers are particularly interested in finding oxygen because the
oxygen in the Earth's atmosphere is continuously replenished, through
photosynthesis, by plant life. Were all life to cease on Earth, our
atmosphere would quickly become devoid of oxygen, which would dissolve
in the oceans and oxidize the surface. Thus, the presence of large
quantities of oxygen in the atmosphere of a distant planet would signal
the likely presence of life there.
NASA's James Webb Space Telescope (JWST), scheduled for launch by the
end of this decade, promises to sniff out the gases of these alien
worlds. Loeb and Maoz created a synthetic spectrum, replicating what
JWST would see if it examined a habitable planet orbiting a white dwarf.
They found that both oxygen and water vapor would be detectable with
only a few hours of total observation time.
"JWST offers the best hope of finding an inhabited planet in the near future," said Maoz.
Recent research
by CfA astronomers Courtney Dressing and David Charbonneau showed that
the closest habitable planet is likely to orbit a red dwarf star (a
cool, low-mass star undergoing nuclear fusion). Since a red dwarf,
although smaller and fainter than the Sun, is much larger and brighter
than a white dwarf, its glare would overwhelm the faint signal from an
orbiting planet's atmosphere. JWST would have to observe hundreds of
hours of transits to have any hope of analyzing the atmosphere's
composition.
"Although the closest habitable planet might orbit a red dwarf star, the
closest one we can easily prove to be life-bearing might orbit a white
dwarf," said Loeb.
Their paper has been accepted for publication in the Monthly Notices of the Royal Astronomical Society and is available online.
Headquartered in Cambridge, Mass., the
Harvard-Smithsonian Center for Astrophysics (CfA) is a joint
collaboration between the Smithsonian Astrophysical Observatory and the
Harvard College Observatory. CfA scientists, organized into six research
divisions, study the origin, evolution and ultimate fate of the
universe.
For more information, contact:
David A. Aguilar
Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
617-495-7462
daguilar@cfa.harvard.edu
Christine Pulliam
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
617-495-7463
cpulliam@cfa.harvard.edu
