Among stars, a water quest
Sir Frederick Martin McNeill
Scientists try to identify likely locales for life in vast universe
02:43 PM CST on Sunday, February 15, 2004
By TOM SIEGFRIED / The Dallas Morning News
http://www.dallasnews.com/sharedcontent/dws/news/healthscience/stories/021604dnlivastrobio.1c52a.html
In politics, it's follow the money.
In Harry Potter, it's follow the spiders.
In the search for life beyond Earth, it's follow the water.
And so on Mars today, two robotic rovers seek signs that the Red Planet was once a wet planet.
Rocks are scraped and scrutinized to see if their innards include minerals that formed with the
help of H{-2}O.
For only if liquid water had ever been around is there any hope that "Martian" was once a
nonfictional noun.
Life, as scientists understand it, needs water the way hockey needs ice. Without water, chemistry
can't get complex enough for life to do all the things it does, like breathing and sex – at least
as life is known on Earth. (There's always some chance for the alien equivalent of field hockey.)
Following the water seems to be the best hope for finding evidence that humans cohabit the cosmos
with other life forms – to discover whether Earth is the universe's only home for life.
"For the first time in human history," writes astronomer Sara Seager, "we are on the verge of being
able to answer this question."
But to actually find that answer, Mars is not world enough. Scientists will have to follow the
water far beyond Mars, beyond the solar system, to planets orbiting faraway stars in other
hospitable ZIP codes of the Milky Way galaxy.
Fortunately, it seems, there are planets, planets, everywhere – and some may possess plenty of
drops of water for life to drink.
There is no life on Mars now – at least, nothing that would qualify even as a contestant for a
reality TV show. Martian water is frozen in the planet's polar ice caps and perhaps elsewhere under
the planet's surface. But the Martian landscape advertises a much moister past. Channels seem to
have been carved by ancient rivers, and broad depressions look for all the world like dried-up lake
beds.
Grayish rust known as hematite, detected from orbit and eyeballed up close by the rover
Opportunity, offers further evidence of a watery history. Hematite isn't ironclad proof of water,
but other minerals nearby may help confirm the belief that water once flowed on Mars. Details
teased out of the minerals by the rover's sensors may tell scientists further tales about the Red
Planet's primordial climate – such as whether the weather was ever suitable for life to survive, if
not thrive.
If the ancient Martian environment was indeed moist – and if future missions find further clues
that life took advantage of it – scientists will have cemented a key piece into the search-for-life
puzzle. They will know that Earth isn't unusual, that it's not the only habitable planet in
history. And if there is more than one, there are probably more than two. Following the water is
all about finding where those other habitable planets are hiding.
Merely a decade ago, the existence of planets orbiting stars other than the sun was an open
question. But in 1995 astronomers announced unmistakable signs of such an "extrasolar" planet.
Since then, many others have been found – nearly 120 by last count – settling forever the question
of how special the sun is. When it comes to planets, the sun is not special at all. Lots of stars
have planets.
True, the planets discovered so far are nothing like the Earth. Most are big and gaseous; the Earth
is small and rocky. And it's the small and rocky (or "terrestrial") planets that astronomers want
to find. But with present-day technology, such planets are much too small to see from Earth.
Actually, the big gaseous planets are too small to see, too. Their presence is detected indirectly,
by changes in the color of a star's light caused by the tugging of a massive planet's gravity.
Detecting a clockwork cycle of such color changes, corresponding to a planet's orbit, reveals
planets around stars dozens of light-years away.
Of the distant planets found so far, most are not good candidates for life.
Habitable planets must occupy a goldilocks region of space near their parent star, not too hot or
too cold for life to originate and evolve. They must be made of life-friendly materials, with ample
water. But many of the known extrasolar planets occupy infernolike orbits very close to their
stars. And nearly all are giant gaseous planets without rocky foundations for oceans or lakes.
But the giant planets might have smaller siblings, the way Jupiter and Saturn are merely the
biggest of a family that includes the Earth.
To test whether known extrasolar systems might include habitable planets, scientists are naturally
following the water. David Underwood, Barrie Jones and P. Nick Sleep of The Open University in
England, for example, have identified "habitability zones" around the 104 stars known to possess at
least one planet – defining a habitable zone precisely as the region around a star where liquid
water could exist on a planet's surface.
The British scientists analyzed dozens of hypothetical stars – ranging from half the mass of the
sun to half again as massive as the sun, with various amounts of chemical ingredients. Computer
simulations determined the water-friendly zone surrounding each "model" star, and known planetary
systems were matched to their best fit among the models. Taking the habitable zone around the real
star to be the same as in its best match, the scientists calculated whether an "Earth" could
survive, given the orbits that the other planets already occupy.
In some systems, a huge planet orbits in or near the habitable zone – bad news for a watery
"Earth." It would either smash into the big planet, or be kicked by the planet's gravity into an
unstable orbit, possibly leading to a fiery death in the central star, or an icy journey out of the
stellar system altogether, the study found. For other known planetary systems, though, such
tragedies did not seem likely. Many seem able to support a habitable planet for a long time.
"About half of the 104 systems could have housed an 'Earth' in their habitable zones for at least
the past billion years," the scientists reported in a paper posted on the Internet.
Further hope comes from studies of the planetary system around the star 47 Ursae Majoris (or UMa),
about 44 light years from Earth. That star possesses two giant planets at a considerable distance,
though not as far as Jupiter and Saturn are from the sun.
47 UMa is about the same mass and temperature as the sun, astrophysicist Manfred Cuntz, of the
University of Texas at Arlington, and colleagues in Germany point out in a paper published last
year in the journal Icarus. They cite several recent studies indicating that not only might a small
planet survive in the 47 UMa habitable zone, but that it could be big enough to exhibit the
tectonic activity that produces continents. The internal churning of tectonics in action also
contributes to cycling substances such as carbon dioxide into the atmosphere, an important part of
keeping the planet habitable. Dr. Cuntz and his collaborators found that an "Earth" at 47 UMa might
be habitable if the star itself is not too bright.
In a subsequent paper, in the International Journal of Astrobiology , Dr. Cuntz and colleagues
found that the best chance for a stable and habitable orbit occurs for a small planet almost
completely covered with oceans. On planets covered mostly with continents, weathering removes more
carbon dioxide from the air, making the planet's surface colder and less likely to be habitable.
All rocks and no oceans, then, would make life's chances pretty dim. Another reason to follow the
water.
Luckily, computer simulations of the planetary birthing process suggest that water worlds would not
be rare.
Sean Raymond and Thomas Quinn (of the University of Washington) and Jonathan Lunine (of the
University of Arizona) tested 42 versions of terrestrial planet formation in the dusty disk
surrounding a young star, varying such features as the mass and orbit of a Jupiterlike planet in
the system. (A large planet's presence influences how many water-containing meteorites would crash
into an embryonic Earthlike planet.) In the 42 tests, more than 100 planets were formed – 43 in a
roughly habitable zone and 11 at a distance from the star very nearly the same as Earth's. Some of
those planets turned out dry in the simulations, but others received enough water for multiple
oceans, the scientists reported in a paper submitted to Icarus.
Ultimately, of course, the only way to find out if earthlike planets really exist is to look for
them. And many elaborate plans are being made to make it so.
One way involves watching stars carefully to see whether their light dims slightly on a cyclic
schedule – the dimming caused by a planet passing in front of it. A NASA mission planned for 2007
will monitor thousands of stars for such eclipselike signals.
Even better, of course, would be direct detection of distant Earths, with actual images. That will
be harder. Glare from the star obliterates the faint light reflected from a small planet nearby.
But astronomers have proposed clever ways to overcome that obstacle.
Telescope mirrors can be arranged, for instance, so that light from the parent star is canceled out
by interference, the way colliding water waves disappear if they meet out of synch. Ideas on the
drawing board for making use of the starlight cancellation trick, known as null interferometry,
include a space-based telescope called FKSI (pronounced "foxy"), under development at NASA's
Goddard Space Flight Center in Greenbelt, Md. If approved for funding, FKSI would be the
"Volkswagen" precursor paving the way for NASA's "Hummer" of planet searchers, the Terrestrial
Planet Finder tentatively scheduled for launch in 2015 or later, says William Danchi, leader of the
Goddard FKSI team.
The European Space Agency is also designing Darwin, a multiple-telescope flotilla of spacecraft for
launch possibly as early as 2014. And some astronomers believe supersized Earth-based telescopes
might be able to join in the hunt.
Success at seeing such a planet, though, would not quite be the same as believing it supported
life. Images would have to be analyzed for the telltale colors of specific chemicals, such as
oxygen, carbon dioxide and, naturally, water. Large amounts of oxygen in molecular form (two oxygen
atoms linked to form an O{-2} molecule) would almost surely be a sign of life.
"There seem to be no nonbiological sources that can continually produce large quantities of O{-2},"
Dr. Seager wrote in a paper published last year in Earth and Planetary Science Letters. It may be
possible to detect patterns of light typical of vegetation, as well.
Although the odds that future searches will find a water-bearing planet may be good, odds are still
odds, never sure things. As with playing the lottery, the best way to win is to buy as many tickets
as possible. Alien life's chances are best if there are many possible planets that might possess
water.
Calculating those odds requires extending the idea of a water-friendly habitable zone farther out
into the reaches of space, to the Milky Way galaxy as a whole. Just as only a small region around
any given star would be temperate enough for water-based life, only some parts of the galaxy would
contain the right ingredients for a habitable environment.
It's not just a matter of water-friendly temperature. The habitable region must be seeded with
heavy elements suitable for forming rocky planets (and providing chemical precursors to life), and
it must be far enough away from stellar explosions known as supernovas that would bathe any such
planets in deadly radiation. And all these conditions must be maintained for a long enough time for
complex life to evolve – perhaps 3 billion years or more.
In a study of these factors published last month in Science, Australian astrophysicists conclude
that the Milky Way galaxy's habitable zone extends from about 20,000 light years from the center of
the galaxy to roughly 30,000 light years away (the Earth, unsurprisingly, sits neatly within that
range, about 27,000 light years from the galactic core.) Although that zone probably contains no
more than 10 percent of the galaxy's stars, that still translates to roughly 10 billion possible
planetary systems.
What's more, the Australian analysis concluded, three-fourths of those stars are older than the sun
(which is nearing its 5 billionth birthday), so there would have been plenty of time for complex
life to evolve billions of times.
So if the galaxy teems with populous watery planets, why do budget-strapped Earth-based astronomers
have to do all the hard work of seeking them? Why don't the aliens send emissaries to Earth? Or at
least an e-mail?
That question was made famous half a century ago by the Italian-American physicist Enrico Fermi.
"Where is everybody?" he asked at lunch one day, alluding to a previous conversation about the lack
of alien visitors.
Since then, more answers have been suggested than extrasolar planets have been discovered. Alien
civilizations may typically discover nuclear energy before space travel and blow themselves up.
Aliens planning to send messages to Earth may have had their budget cut. Aliens choose not to
bother backward civilizations. Aliens try to communicate but are too advanced for us to understand
them. Or not advanced enough.
Or maybe aliens haven't visited because they simply don't exist.
Half a century ago, when Fermi posed the question, that was a logical answer. But today it doesn't
wash. The water trail leads to the conclusion that populated planets elsewhere in the galaxy are
not merely possible, they are likely.
Mars may be barren today, but elsewhere in the cosmos water must still be flowing. And if
astronomers continue to go with the flow, they will most likely learn that if the Earth has no
monopoly on water, it has no monopoly on life.
E-mail tsieg...@dallasnews.com