Artist's concept of Kepler-20e: NASA's Kepler mission has discovered the first Earth-size planets orbiting a sun-like star outside our solar system. The planets, called Kepler-20e and Kepler-20f, are too close to their star to be in the so-called habitable zone where liquid water could exist on a planet's surface, but they are the smallest exoplanets ever confirmed around a star like our sun. (Nasa)

In a milestone hailed by scientists as a key step toward finding another Earth-like world, astronomers Tuesday announced the discovery of two blazingly hot planets roughly the size of Earth some 950 light years distant.

The discovery “demonstrates for first time that Earth-size planets exist around other stars, and that we can detect them,” said Francois Fressin, an astronomer at the Harvard-Smithsonian Center for Astrophysics who led the discovery team.

The two planets orbit a star much like our sun, but they whiz around it so fast and so close that their surfaces sizzle like frying pans.

“They’re way too hot to be anything like our own Earth,” said Sara Seager, a planet hunter at the Massachusetts Institute of Technology and a member of the discovery team.

The implication: No life — at least life as we can conceive it — is possible on the new planets.

Still, finding these hot Earth-sized planets is “seriously cool,” said Lisa Kaltenegger, who studies so-called exoplanets at the Max Planck Institute in Heidelberg, Germany, and was not involved in the research. “These discoveries are a great technological step forward.”

The planets were announced Tuesday in the journal Nature and during a NASA teleconference.

Detected by NASA’s Kepler space telescope, the two planets, dubbed Kepler 20e and Kepler 20f, are almost certainly rocky like Earth and not gaseous like Jupiter, Kaltenegger said.

The smaller planet, Kepler 20e, is about the size of Venus but much closer to its star, zooming around it every six days. An Earth year, by contrast, is 365 days.

The larger planet, Kepler 20f, is just three percent larger than Earth. “It’s the first Earth-sized planet” ever detected orbiting another star, Seager said. “It is a big milestone.”

Kepler 20f is a bit farther out from its star, completing an orbit about every 20 days. Its surface temperature is hotter than a pizza oven — about 800 degrees Fahrenheit.

The two planets nestle in among three other larger planets tightly circling the star Kepler 20.

“It’s a beautiful planetary system,” said Dimitar Sasselov, a planet hunter at the Harvard-Smithsonian center and a member of the discovery team.

But it’s also a puzzling one. None of the five planets lie within the so-called habitable zone, the narrow band of space around a star where water can exist as liquid. Instead, all five planets hug their star, orbiting closer than Mercury is to the sun.

And, unlike our solar system, the two newly-found rocky planets are interspersed with three larger, gassy, Neptune-like planets.

“The architecture of that solar system is crazy,” said David Charbonneau of Harvard University. “In our solar system, the two different kinds of planets don’t mingle. This is the first time we’ve seen anything like this.”

The finds mark a key moment in the accelerating search to bag and tag planets outside our solar system. Since the first such detection in 1995, multiple teams employing ground and space telescopes have found more than 700 planets orbiting other stars, according to an online catalogue.

The message, says Seager, is simple: Planets abound wherever we look. “We think every star has planets,” she said.

NASA launched the $600 million Kepler space telescope in 2009 on a mission to find other Earth-like planets. So far, the telescope has found 33 confirmed planets and 2,326 possible planets, but they are all too big or too hot to qualify as Earth-like. The telescope detects planets by staring at 150,000 stars near the constellation Cygnus. When light from a star dims, or winks, it indicates a possible planet passing by. If Kepler sees the same wink three times, astronomers infer a planet. The time that passes between winks indicates the planet’s orbital period, or year.

Earlier this month, Kepler scientists announced a planet square in its star’s habitable zone. Dubbed Kepler 22b, that planet is about 1.4 times as wide as Earth, likely too large to host a rocky surface. “It’s too big, we think, for life,” Seager said.

The next milestone for Kepler will be the big one: The detection of an Earth-sized world with a surface temperature just right for life. Kepler scientists are confident they will soon spot such an Earth 2.0.

“One of these days — whether next year or two years from now — Kepler will confirm a true Earth analog,” Sasselov said. “And that will be a historic moment.”

When asked whether the Kepler team planned to give the newly found Earth-sized planets a catchier name, Sasselov balked. “Everybody wants pretty names,” he said. “But what do we do? There will be thousands of these planets.”