There are about ten thousand billion billion habitable
planets in the observable universe, and some of these Earth-like worlds could
be found by a mission set to launch early next month, a leading
planet-formation theorist now speculates.
Alan Boss, astrophysicist at the Carnegie Institution in
Washington, D.C., and author of "The Crowded Universe" (Basic Books),
published this month, came up with that rough number by estimating there is
about one habitable
planet around every sun-like star in the galaxy, of which there are about
10 billion, and multiplying that by the number
of galaxies in the universe (about 100 billion).
This
result is inexact of course, so give or take a power of ten or so, Boss said,
which is standard for these types of estimates in astronomy.
"Based
on what we already know, the universe is going to turn out to be chock full of
habitable planets (i.e. Earth-like worlds), and therefore life is likely to be
widespread," said Boss, who discussed these estimates with a group of
reporters last weekend in Chicago at the annual meeting of the American
Association for the Advancement of Science.
The
promise of 'super-Earths'
To
date, precisely zero of these other Earths have been found. Technology simply
has not allowed their discovery, presuming they exist. But astronomers are
closing in. In the past nearly 15 years, more than
300 planets have been found around stars beyond the sun.
Three classes of planets have
been found, for the most part — Jupiter-like gas giants, Neptune-like icy
planets and hot "super-Earths."
Such super-Earths, such as one reported by the Carnegie Institution's Paul
Butler in 2004 around Gliese 436 and another reported the same year by Barbara
McArthur of the University around 55 Cancri, have masses of about five to 10
times that of Earth and exist around one-third of all nearby stars like our
sun, Boss figures. This estimate is based on the results of ongoing
planet-search efforts using the gravitational tug of planets on stars to detect
worlds, called the Doppler approach, he said.
Most of these super-Earths are too hot to support life,
but Boss thinks there are warm super-Earths, with longer period orbits and more
suitable for life. Examples are two warm or cool super-Earths reported in 2007
by Stephane Udry and his colleagues on the Geneva Observatory to be orbiting
Gliese 581.
And, some of the icy planets might turn out to be rocky
planets similar in composition to Earth, only more massive, Boss said.
"We already know from folks who have been finding
planets around other stars that most stars have planets," he said, adding
that "simply from a theoretical ground of understanding how stars form,
it's almost inevitable that they should end up having disks around them which
should end up forming planets. So we expect them to be there from the point of
view of theory as well."
Kepler will test it
Boss's claims will be tested by NASA's Kepler mission, a
1-meter-diameter space telescope set to launch March 5 from Cape Canaveral
aboard a Delta 2 rocket. Among other tasks, Kepler is designed to count the
number of Earth-sized and larger planets in the habitable zone around stars
like the sun. Results should come in the next three or four years.
Kepler will detect planets using the "transit
technique," which involves inferring the presence of a planet by detecting
the dimming
of star light caused by the passage of the orbiting planet in front of the
star.
In the search for Earth-like planets, Kepler will be
racing against the French-led CoRoT (Convection, Rotation and Planetary
Transits) mission, a 27-centimeter-diameter space telescope launched in 2006.
CoRoT already has found the smallest planet (COROT-Exo-7b) ever detected
orbiting a sun-like star. Twice the size of Earth, the exoplanet's temperature
is so high that it could be covered with lava or water vapor, CoRoT scientists
said.
"We'd be astonished if Kepler and CoRoT did not find planets, because they
are already finding them," Boss said. Kepler might find more planets than
CoRoT because its telescope has a larger diameter (therefore it is more
sensitive) and it will survey a larger patch of sky, he said.
"The fact that we can find [roughly Earth-sized
planets] already implies that we are just seeing just the tip of the iceberg.
There might be many more Earths out there waiting for us to find," Boss
said.
And some of these could be found relatively close to
Earth, he said.
"There are something like a few dozen solar-type
stars within 30 light years of the sun," Boss said. "I would think a
good number of those, perhaps half of them, will have Earth-like planets. So I
think there are very good chances that we'll find some Earth-like planets
within 10, 20, 30 light years of the sun."
Once the frequency of habitable-Earth-like planets in our
neighborhood of the galaxy is known, scientists will be better able to design
space telescopes capable of imaging those worlds and detecting evidence of the
molecules necessary for life, such as water and oxygen, and possibly even those
created by life, such as methane, Boss said.
What took you so long?
Physicist Enrico Fermi, who lived in the first half of
the 20th century and is known for developing the first nuclear
reactor, thought there was intelligent life beyond Earth, but famously wondered
why we haven't heard from it yet (this question is called Fermi's Paradox),
Boss writes in his book.
"The answer to that question ranges from practical
to suicidal," Boss told reporters Saturday.
The suicidal version: "Maybe it means civilizations
which are capable of sending us radio signals just don't last that long. ... Do
we really think our civilization is going to last 1 billion years?"
The practical version: "There is a low probability
of success [as with the SETI effort], but if you find something you have an
immensely important finding."
Traveling to even the closest star orbited by a planet
with habitable life could take us hundreds of thousands of years, he said.
Meanwhile, the impact of CoRoT and Kepler finding
Earth-like planets around sun-like stars will be huge, Boss said.
"Once we find the first one we'll have made the
point that they really are there," Boss said. "Just by finding the
first one we'll able to immediately say, 'Well if we found one around a nearby
star, just by multiplying the volume of the stars searched versus the ones that
have not been searched, we can infer there must be billions and billions more
of them just within our own galaxy.' So finding the first one will have
enormous implications for how many there are in the entire galaxy as well as in
the entire universe."
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