This NASA/ESA Hubble Space Telescope image above shows the center of globular cluster M4. It contains several tens of thousands stars and is noteworthy in being home to many white dwarfs—the cores of ancient, dying stars whose outer layers have drifted away into space.
In July 2003, Hubble helped make the astounding discovery of a planet called PSR B1620-26 b, 2.5 times the mass of Jupiter,
which is located in this cluster. The PSR B1620-26 system lies around
5,600 light years away in globular cluster M4, boxed in green, directly
west of red supergiant star Antares in Constellation ScorpiusIts age is
estimated to be around 13 billion years—almost three times as old as
the Solar System! It is also unusual in that it orbits a binary system of a white dwarf and a pulsar (a type of neutron star)
The existence of a 13-billion year old planet, if in fact it still
exists, highlights the fact that our Solar System exits in a universe
that is estimated to be between 13.5 and 14 billion years old. Some
astronomers, including Sir Martin Rees of Cambridge University believe
that there could be advanced civilizations out there that have existed
for 1.8 gigayears (one gigayear = one billion years) and longer.
A few intrepid astronomers have concluded that the most productive to
look for planets that can support life is around dim, dying stars white
dwarfs so prevalent in M4.
"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. 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.
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.
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