Note: This is a “classic” Interesting Thing of the Day article from over 10 years ago. It has not been edited recently, so it may contain broken links, outdated information, or other infelicities. We plan to eventually update or retire most classic articles, as time permits.

Author’s Update: The Kepler Mission finally launched on March 6, 2009, and has already found several planets. For current updates on its status, check the mission’s Official Web Site.

Ever since I was a kid, I’ve been a fan of Star Trek in its various incarnations. I have no trouble suspending my disbelief in the seemingly incredible technology of the future, and I can even accept that somehow, mysteriously, humanoid beings all over the galaxy speak English. But there are some recurring Trek themes that boggle the mind because they seemingly defy the laws of statistics. For example, we viewers are expected to believe that upon encountering any alien race, there is a 35% probability that some hot alien chick will fall in love with the captain of the Enterprise within an hour. Well…I don’t know that the probability is 35%, but it’s certainly a few orders of magnitude higher than what common sense tells me. Likewise, when our heroes encounter a new planet, almost without exception they observe with feigned surprise that it’s a “Class M” planet—Trek shorthand for “able to sustain human life.”

Of course, any planet on which human actors are going to be filming a TV show would pretty much have to support human life, wouldn’t it? But in the real universe, it’s hard to imagine that there would be very many worlds so similar to Earth that humans (or human-like beings) could live there comfortably. After all, a lot of very specific criteria would need to be met. The atmosphere must have just the right mixture of gases—and must provide protection from stellar radiation. The temperature must be within the range humans can tolerate. The gravity must be strong enough that we stick to the surface, but not so strong that we can’t walk. There will have to be some liquid water somewhere on the planet. And there must be no environmental toxins or hazards that could cause us serious harm. In short, there are a lot of variables that have to be just right, and you’re not going to find that winning combination just anywhere. You need, in fact, to find a planet that’s very, very similar to Earth in terms of its size, distance from its sun, rotational speed, and so on.

So what are the odds, really, of finding other planets like that? The Kepler Mission, a project currently in development by NASA and the SETI Institute, aims to find out.

This Little Light

Planets outside our solar system reflect far too little light to be seen from Earth. So we only know of their existence inferentially. For example, a technique called Doppler spectroscopy measures the apparent shift in a star’s color when a large planet’s orbit takes it directly between the star and Earth. Using this technique, astronomers have been able to detect more than 100 gas giants (similar to Jupiter) orbiting other stars. However, smaller planets (on the order of Earth’s size) cannot be detected this way because the color shift is too slight. What we can measure, however, is the tiny decrease in brightness as a smaller planet passes in front of a star. Or we could, if we paired an extremely sensitive digital sensor with a powerful telescope—and put it in space, where our atmosphere wouldn’t interfere with the readings. This technique is called transit photometry, and that is what the Kepler Mission is designed to do.

The Kepler Mission is currently scheduled for launch in 2007. For four years, it will unblinkingly watch a field of about 100,000 stars, looking for those characteristic dips in brightness. This is tricky business—the decrease in brightness has been compared to that caused by a gnat flying in front of a car’s headlight. And of course this temporary dimming must be observed at regular intervals (say, once a year) to prove that it really was caused by an orbiting planet.

Planes and Planets

There’s another problem too. Even if a planet exactly like Earth were orbiting a certain star, there’s no guarantee that the plane of its orbit would be in alignment with the Kepler telescope. Astronomers estimate that for any given star, there’s a 1 in 200 chance that a planet’s orbit will be just right to be detected using this method. Statistically speaking, it should be possible to detect small planets—if they exist—in about 500 other solar systems. Needless to say, not all stars are expected to have Earth-like planets in orbit around them, but the Kepler Mission’s designers hope to find at least 50, and possibly hundreds, of potentially habitable planets.

Still, all this will really tell us is whether there are stars with planets of roughly the right mass and orbit. Transit photometry can’t tell us anything about a planet’s atmosphere—a crucial part of its habitability. For that, we’ll have to wait until at least 2012. Another of several proposed NASA projects, called the Terrestrial Planet Finder, would use multiple spacecraft with much larger telescopes and more sophisticated techniques to collect detailed data on the atmospheres of potentially habitable planets in other solar systems. Even if one of the missions succeeds, we still won’t know if those planets have any hot alien chicks, but we have to leave something for Captain Kirk to discover a few centuries from now. —Joe Kissell

More Information

To learn more about the Kepler Mission, visit these Web sites:

For more information about the Terrestrial Planet Finder project (which has now been postponed indefinitely), see Terrestrial Planet Finder at NASA’s Jet Propulsion Laboratory or

Yet another of the many techniques for finding Earth-like planets is called microlensing; see Smallest extrasolar planet revealed by microlensing in New Scientist.


div>Cosmic Company: The Search for Life in the Universe by Seth Shostak and Alex Barnett, Search for Life on Other Planets by Bruce Jakosky, or <a href=”””>cover art

Looking for a down-to-earth book on the search for extraterrestrial life? Try Cosmic Company: The Search for Life in the Universe by Seth Shostak and Alex Barnett, Search for Life on Other Planets by Bruce Jakosky, or Here Be Dragons: The Scientific Quest for Extraterrestrial Life by David W. Koerner and Simon Levay.

Astronomers have found an “Earth-like” planet using terrestrial telescopes according to this article on Earth-like planet discovered 50 light-years away. However, this planet is about 14 times the size of Earth and far too hot to be habitable.