Another Earth?

"Planet e" in the foreground of this artist's rendering is twice the mass of Earth and closest to its sun. "Planet d," the blue dot, is the fourth planet out and may be at the right distance to have liquid water, scientists say.

hen the National Aeronautics and Space Administration (NASA) launched its Kepler telescope in March, the space agency hoped to find the first Earthlike planets outside our solar system.

As NASA waits for Kepler to start sending back images and data, a group of European astronomers is stealing NASA's thunder.

On April 21, a team of Swiss and French researchers working at the European Southern Observatory in Chile reported they had discovered the smallest and most Earthlike planets ever seen beyond our solar system.

Astronomers found the planets in a solar system called Gliese 581. It's a system of four planets orbiting a dim red star called a red dwarf. Gliese 581 is about 20 light-years from Earth.

The team has been studying Gliese 581 for four years. In 2007, it discovered three planets in the systems. Researchers named the planets 581b, 581c, and 581d. At the time, scientists ruled out the possibility of any liquid water or life on any of the three planets.

But now the researchers have changed their minds about planet 581d. Their recent research tells them that 581d is in just the right orbit and position for liquid water to possibly be there. That would mean the planet could sustain life.

According to the researchers, planet 581d is about seven times as big as Earth, too big to be just rock. So they think there could be a liquid ocean on its surface. Stephane Udry, one of the researchers, says the planet is the first serious "water world candidate" observed outside our solar system.

Another discovery made by the astronomers is the Gliese 581 system's fourth planet, called 581e.

This planet is far too close to its sun to sustain life. Its orbit puts it so close to its sun that the planet would be super hot and bombarded with radiation.

But the planet's size has astronomers excited. Planet 581e is similar in size to Earth. According to scientists, it could be about 1.9 times the size of Earth.

Scientists usually discover planets that exist outside our solar system—called exoplanets—using what they call the wobble method. They will watch a star, and if there is a wobble in its light, that might mean that a planet's gravity is influencing the star's gravitational pull. More than 340 planets have been discovered that way since 1995. Most have been about the size of Jupiter.

But Earthlike planets are too small and have too weak a gravitational pull to be found using the wobble method. Instead, astronomers use disruptions in starlight to zero in on these planets. It's a much more difficult task.

But by discovering these small planets, scientists have taken the first steps toward finding an actual Earthlike planet in another solar system.

"I think it's an amazing discovery," said Jon Jenkins, a principal investigator on NASA's Kepler mission. "There should be many more planets similar to these. I think we're seeing just a glimpse of what's out there."

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