This artist’s impression made available by the European Southern Observatory on Tuesday, Oct. 16, 2012 shows a planet, right, orbiting the star Alpha Centauri B, center, a member of the triple star system that is the closest to Earth. Alpha Centauri A is at left. (L. Calcada/AP)

A star nearly next door to our sun — in galactic terms — is home to a hot little planet about the same size as Earth, scientists announced Tuesday.

This overheated world hugs the star Alpha Centauri B, zipping around it every three days.

It’s the nearest so-called exoplanet yet discovered. The planet’s Earth-like size and orbit around a sunlike star make it a “landmark discovery,” said Stephane Udry of the University of Geneva, leader of the research team.

“This is in our back yard,” said Gregory Laughlin, an astronomer at the University of California at Santa Cruz and a member of a rival team also searching the Alpha Centauri system for planets.

Alpha Centauri B is slightly smaller, dimmer and more yellow than our sun. The new planet, dubbed Alpha Centauri Bb, is much closer to that star than Mercury is to our sun.

Alpha Centauri B is visible only from the Southern Hemisphere, so the research team studied it with instruments at the European Southern Observatory in Chile.

They detected the planet indirectly, by seeing Alpha Centauri B wobble at a speed of about one mile per hour — a sign of a small planet tugging on it.

Detecting it was tricky, requiring 450 nights of observation over four years.

The new planet’s Earth-like mass marks it as a rocky body, not a gas planet like Jupiter, said Xavier Dumusque, a University of Geneva astronomer and the lead author of a paper published online in the journal Nature describing the find.

Dumusque said it’s likely that the planet’s surface “is not solid but more like lava — like a ‘lava planet.’ ”

One expert said the finding needs to be confirmed. “Only if other analyses come to the same conclusion can we be sure that this planet exists,” astronomer Artie Hatzes wrote in a companion article in Nature.

Udry, in response, said there is less than one chance in 1,000 that the discovery is a phantom of the team’s data.

As the closest stars to our sun, some four light-years distant, the Alpha Centauri system has long intrigued astronomers. Unlike our solar system, the system contains three stars locked in a gravitational dance.

In the 1990s, astronomers listened to the Alpha Centauri system for alien radio broadcasts but heard nothing, said Seth Shostak, senior astronomer at the SETI Institute in California. Any aliens on the new planet “would have to be devilish and enjoy hot weather,” he said, adding that the SETI (Search for Extraterrestrial Intelligence) project will probably take another listen across a broader range of radio channels in case other, more habitable planets also lurk in the system.

Since 1995, astronomers have listed 842 planets around other stars, according to one catalogue, revealing that most stars have planets. They’ve discovered a likely “diamond” planet, possible ocean worlds and huge, Jupiter-like behemoths too hot to sustain any conceivable life.

They have yet to find their biggest quarry, an “Earth 2.0” — an Earth-size planet orbiting a sunlike star at just the right distance for liquid water.