Out There: Billions and Billions of Habitable Planets

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 20^th 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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