May 16, 2013
Scientists searching for signs of life
beyond our solar system should keep an open mind, for planets very
different than Earth may well be habitable, a prominent researcher
While it may seem natural to zero in on "alien Earths," such a
narrow focus would exclude many potentially life-supporting
exoplanets, whose diversity
continues to astound astronomers, says Sara Seager of MIT.
And researchers can't afford to be so picky, she adds, since they'll
be able to get in-depth looks at just a handful of alien worlds for
the foreseeable future. [9
Exoplanets That Could Host Alien Life]
"The number of planets that we're
going to be able to see in our lifetime - and look at their
atmospheres for signs of lifeó is so small that we're forced to
be open-minded," Seager told SPACE.com.
Seager discusses exoplanet habitability
in a review article published online today (May 2) in the journal
Scientists discovered the first alien planet around a sunlike star
in 1995. Since then, the tally has grown to more than 700 (or more
than 800, depending on whose list is consulted), with thousands more
candidates waiting to be confirmed by follow-up observations.
Some of these alien worlds are broadly similar to planets in our own
But many others are truly alien -
enormous "hot Jupiters" that whip around their parent stars at
extremely close range, for example, or "rogue planets" that cruise
through the cold depths of space alone, with no parent star.
"If there is one important lesson
from exoplanets, it is that anything is possible within the laws
of physics and chemistry," Seager writes in the Science article.
"Planets of almost all masses, sizes
and orbits have been detected, illustrating not only the
stochastic nature of planet formation but also a subsequent
migration through the planetary disk from the planetís place of
[The Strangest Alien Planets]
Intriguingly, a number of planets have
been spotted orbiting within the so-called "habitable zone" - that
just-right range of distances from a star where
liquid water is possible on a world's surface.
(Water is required for life as we know
it here on Earth and has thus spurred astrobiologists to "follow the
water" on other planets, Seager writes.)
Just where this habitable zone lies for each planet depends on a
number of factors, most crucially its host star's brightness and the
planet's atmospheric makeup.
"It's really all about the
greenhouse gases," Seager told SPACE.com. "The greenhouse gases
are like a blanket that moderates the temperature at the
The conventional definition of the
habitable zone assumes a roughly Earth-like atmosphere, dominated by
nitrogen, carbon dioxide and water vapor.
But the huge diversity of alien worlds
argues for a new way of thinking, writes Seager, who literally wrote
the book on exoplanet atmospheres ("Exoplanet Atmospheres: Physical
Processes," Princeton University Press, 2010).
For example, large and/or chilly alien worlds could conceivably hang
onto their gaseous molecular hydrogen, which long ago escaped from
small planets such as Earth, Venus and Mars.
Hydrogen is a powerful greenhouse gas that could make liquid water
possible on a number of worlds far beyond the outer edge of the
traditional habitable zone - and perhaps even on seemingly frigid
rogue planets, Seager writes.
Similarly, the habitable zone may extend inward, toward the host
star, on "dry" rocky planets whose atmospheres have much less water
vapor than Earth's does. So it's best to consider alien planets'
potential to support life individually, on a case-by-case basis,
Seager and others stress that a better understanding of exoplanet
habitability is key to the next phase of the alien life hunt, which
seeks to search promising candidates' atmospheres for water vapor
and gases that may have been produced by life.
Astronomers have already scanned the air of a few dozen planets
using NASA's Hubble Space Telescope and other instruments, Seager
said. But those were hot Jupiters with big, puffy atmospheres -
relatively easy targets that aren't intriguing from an
Scientists plan to do the same with smaller, potentially habitable
worlds soon, Seager said.
They'll use the
Transiting Exoplanet Survey Satellite
(TESS), which NASA recently approved for a 2017 launch, to identify
promising candidates relatively close to Earth.
James Webb Space Telescope (JWST
- which is scheduled to blast off in 2018) will follow up, getting
an in-depth look at these worlds' air.
Though JWST is designed to be incredibly powerful, the $8.8 billion
instrument will probably only be able to investigate the atmospheres
of exoplanets that lie within a few tens of light-years from Earth,
Seager said she hopes her review article in Science (Exoplanet
Habitability) helps her fellow astronomers make the most
of this small pool of observable candidates.
"I hope it gets people to realize
that so many types of worlds could be habitable, and that our
chance of finding one is higher when we accept that," she told