SPACE EXPLORATION

Exo-Earths—the search for worlds like our own

Focus is on finding planets with water and air at the right distance from a stable parent star.


Closing in

Kepler searches for planets among the stars.

So far, no such world has been found among the 400 known planets orbiting distant stars. Most of them are far too massive, and their orbits tend to be highly eccentric rather than circular. Also, most of the planets detected early on orbit very close to their star, placing them well on the inside of the HZ, with surface temperatures in the hundreds or thousands of degrees Kelvin.

As detection techniques improved, planets were detected further away from their stars as well, but most of these were well outside the HZ, and therefore too cold to support life as we know it. And yet astronomers are getting closer.

The sensitivity of the popular RV technique has improved to the point where it can detect a star moving at a speed of only 1 meter per second—similar to a leisurely stroll. This is still not sufficient to detect the effects of an orbiting Earth-sized planet, which would require a sensitivity around 10 times greater, or the speed of a slow crawl.

But with technology advancing rapidly, even that astonishing sensitivity appears to be within reach in the years ahead. Another method that is becoming increasingly important in the search is “transit photometry,” which uses highly sensitive photometers to measure minuscule changes to a star’s luminosity.

If a slight drop in a star’s brightness occurs at regular intervals and lasts a fixed length of time, it is a strong indication that a planet is passing, or “transiting,” right in front of it. The Kepler mission, launched in Mar 2009, will look for just such transit events in a region of space containing 140,000 stars.

By tracking the same stars for 3 years straight, is expected to discover a slew of Earth-sized planets. Whereas the RV method provides an estimate of a planet’s mass, transit photometry measures a planet’s diameter.

When taken together, these 2 measurements provide the planet’s density, which is a crucial indication of its chemical composition. This is why, when a planet is discovered by one method, there is always an effort to follow up with the other.

It is not always successful, since only a small portion of planets detected by radial velocity actually transit their star. At the same time, the most interesting planets that scientists expect Kepler to find will have too low a mass to be tracked by radial velocity.

But as detection technology continues to improve, more planets of lower and lower mass will undoubtedly be found, their diameter measured and their density recorded. The most Earthlike worlds discovered to date are probably the 4 planets orbiting the red dwarf Gliese 581, 20.5 light years from Earth.

One, designated Gliese 581e, orbits very close to its star and is searingly hot, but at only 1.9 Earth masses it is the smallest exoplanet found to date. Another, designated Gliese 581d, is about 7 Earth masses, and could be covered with ice, like our own neighbor Neptune.

Since it orbits within its star’s HZ, however, where liquid water is stable, it is quite possible that it is completely covered by an ocean, making it the very first “water world.” But it is the planet that orbits between these 2 that most intrigues scientists: Gliese 581c is 5 times the mass of Earth and is almost certainly a rocky planet like our own.

Most importantly, this planet orbits within its star’s HZ, or very close to it. A low-mass rocky planet where liquid water is stable? This might not yet be an exo-Earth, but it’s clearly getting close.

There is little doubt that astronomers are now hot on the trail of the first exo-Earth, and closing in. No one knows how long it will be before a world like our own is found deep in space—but, once made, the implications of the discovery would be staggering—the splendid isolation of our world, which has lasted since the dawn of human consciousness, will come to an end.

Amir Alexander is a science writer for The Planetary Society and teaches the history of science at UCLA. His most recent book is Duel at Dawn: Heroes, Martyrs, and the Rise of Modern Math­ematics (Harvard University Press, 2010).

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