Have Earthlike Planets Really Been Found?

When will we find the first "earthlike planet" in the galaxy?

According to some mainstream news reports we have found them already - again and again and again.

The problem is one of semantics. If an exoplanet is close to the same mass of Earth it's called "earthlike" in press releases and the news media.

The one that got lots of press a couple weeks ago was a world called CoRoT-7b, located 500 light-years away. The planet tips the scales at 4.8 Earth masses and is a little less that twice Earth's diameter and the least massive exoplanet to date that has been found orbiting a normal star.

Artie Hatzes, director of the Thuringer observatory in Germany said, "it's as close to something like the Earth that we've found so far. It's just a little too close to its sun."

Little too close? How about surface temperatures of more than 3,600 degrees Fahrenheit? Instead of an ocean of water it would have an ocean of molten rock. Why so hot? The planet has migrated so close to its star it completes an orbit in just 20 hours. It is 23 times closer to it's star than Mercury is to our sun.

This is no place to call home.

The first planet found in a star's habitable zone – where liquid water could remain on the surface - is Gliese 581d. It has gotten the most press as being earthlike, thought it is larger at approximately 8 Earth masses. It is likely a super-Earth covered in water.

Largely as a publicity stunt, the Australian Cosmos Magazine

collected 26,000 messages from around the world to transmit across the 20 light-years to the Gliese 581 system, should anyone be living on this planet. The NASA/CSIRO Canberra Deep Space Communication Complex at Tidbinbilla, Australia sent the signal out on August 28.

NASA's Kepler observatory is billed as looking for the first earthlike planets. But reporters sometimes overlook the fact that all this means is that Kepler will measure a very faint dip in starlight when the planet passes in front of its parent star. This is called a transit.

Kepler's sampling of over 100,000 stars will yield statistical information about the abundance of small planets in the habitable zones of sunlike stars.

There won't be any pictures of these planets.

They will presumably be rocky, but we won't know anything about the chemical makeup of their atmospheres or surfaces. And we never will because they are too far away from Earth in Kepler's survey starfield that encompasses planets strung along a 1,000 light-year corridor.

If alien astronomers monitored our sun with their own version of Kepler they would find Earth.

But they would also note that Venus and Mars are near the habitable zone too. However, each of these planets is markedly different from one another.

Venus is lifeless because of its runaway greenhouse effect, and Mars is is deep into a planet-wide ice age.

Therefore, astronomers, their press officers and reporters are jumping the gun crying, "Earth" with every exoplanet discovery of a nearly Earth mass planet.

The defining observation is probably 20 or 30 years off.

This will be feasible when we have a larger space telescope with a 16 meter-diameter mirror. It will be powerful enough to yield detailed spectral fingerprints of a sample of nearby inhabited Earth-mass planets in the "Goldilocks" zone.

It's a long shot but NASA's James Webb Space Telescope scheduled for launch in 2014 might eek out a biosignature spectrum from a very nearby exo-Earth candidate. Because Webb's mirror is just 6.5 meters across, astronomers will need to add up many transits to assemble a clean spectrum. Even for stars as close as 20 light-years away this means following hundreds of transits by the same planet.

An earthlike world orbiting a sunlike star would undergo a 10-hour transit once every year. Accumulating 100 hours of transit observations would take 10 years! However an Earth orbiting in a red dwarf star at the closer-in habitable zone would transit every 10 days. Accumulating 100 hours of transit observations would take less than three years.

So if astronomers get really, really lucky – and tenacious – the first bona fide earthlike might barely be within JWST's grasp.