Artist's impression of the planet around Alpha Centauri B. Our own Sun is visible to the upper right. (Source: ESO/L Calcada)
No need to brush up on extraterrestrial etiquette quite yet, however. The planet, which flies around its parent star 10 times closer than Mercury orbits the Sun, which means its surface temperature would be more than 2000°C - far too hot for liquid water to exist on the surface. Water is believed to be necessary for life.
But the newly found planet orbiting Alpha Centauri B, a Sun-like star roughly 25 trillion miles away, could have better-positioned siblings.
"From statistical studies, low-mass planets are very frequently found in multiple systems," says lead researcher Xavier Dumusque, with the University of Geneva in Switzerland.
So far, scientists have only ruled out the possibility of massive planets with orbital periods of 200 days or less around Alpha Centauri B, so that leaves plenty of room for the detection of low-mass planets in the star's so-called "habitable zone" - the distance where water can exist on a planet's surface, says Dumusque.
Planets positioned the same distance as Earth is to the Sun would take 365 days to orbit a parent star of the same type and size as the Sun. Alpha Centauri B's newly found world circles in just 3.2 days, but the star is roughly half the size of the Sun, which puts its habitable zone about where Venus is in our solar system. Venus orbits in 225 days.
Habitable zonesScientists already have found nearly 800 planets beyond the solar system, about 10 per cent of which are considered low-mass worlds, meaning they are up to about 10 times the size of Earth or smaller.
"Most of the low-mass planets are in systems of two, three, up to six or seven planets, so finding in our closest neighbour one Earth-mass planet ... opens a really good prospect for detecting planets in the habitable zone in the system that is very close to us," says astronomer Stephane Udry, with Geneva University in Switzerland.
"In that sense, it is a landmark," Udry said.
Scientists using Europe's HARPS telescope spent four years trying to ferret out telltale signs of a small planet's gravitational tug on light coming from Alpha Centauri B.
The measurement is difficult because of variations in the star's light caused by other phenomenon, such as flares and magnetic storms, similar to sunspots on the sun.
"Trying to extract a signal that you are interested in when it is in the presence of "noise" - in this case the variability of the star -- is difficult. One has to apply special analysis methods and tricks. The real challenge, in this particular case, was in how to analyze the data," says astronomer Artie Hatzes, with Thuringian State Observatory in Tautenburg, Germany.
"I still have my doubts," Hatzes adds. "Even though there is clearly a signal in the data at 3.26 days, the nature of this is still open to debate."
More data - and more sensitive instruments - will nail down whether the planet actually exists or not, and if it has any siblings.
"Everything we know about this system so far is extremely tantalising," says astronomer Greg Laughlin, with the University of California in Santa Cruz. "This is our backyard and to find out that planet formation did occur there is just extraordinarily exciting."
The research appears in this week's Nature.