Earth-like rocky planets could be hiding just a few light years away in our closest stellar neighbours. That’s the bold claim by a team of American astronomers at the University of California, Santa Cruz, who also argue that dedicated study of the three-star Alpha Centauri system some four light-years away using existing astronomical techniques could potentially unveil terrestrial exoplanets in as little as five years.

The possible existence of planets beyond our Solar System, and hence the existence of extraterrestrial life, has been the subject of speculation for much of recorded history. Greek philosophers such as Thales argued for a universe full of other planets, and in 1600 the Catholic Church even burnt the Italian astronomer Giordano Bruno — a contemporary of Galileo — at the stake for publishing a book arguing for an infinite universe in which every star has a solar system and in which intelligent life populates the planets.

However, it took until 1993 for astronomers to discover the first so-called exoplanet, thanks to the difficulty in detecting a planet’s reflected light amid the glare of its parent star. Since then over 200 exoplanets have been discovered, many by examining the “wobble” in a star caused by the shifting centre of gravity of an orbiting planet, a technique known as Doppler detection. Nevertheless, only a handful of the exoplanets discovered are believed to have a density high enough to be rocky, and only one of these — found in mid 2007 by the ESO’s 3.6-m telescope at La Silla, Chile — has presented even the possibility of having liquid water.

In five years we could detect entire neighbouring planetary systems Javiera Guedes, University of California, Santa Cruz

Seeing our neighbours

Now, Javiera Guedes and her colleagues argue that the same technique could be applied to our nearest neighbour — the Alpha Centauri star system — to find Earth-like planets. According to them, computer simulations reveal that terrestrial planets are likely to have formed around Alpha Centauri B and could even be within the star’s “habitable zone” where liquid water can exist on a planet’s surface (Astrophys. J. in publication).

“We simulated a proto-planetary disk around Alpha Centauri B over 200 million years and in all simulations were able to form planets of 1-2 Earth masses,” explains Guedes. “Forty percent of these Earths lie in the so-called habitable zone of the star. If these planets do exist, we can observe them using modest resources, such as a one-metre telescope.”

The researchers believe Alpha Centauri B is among the best candidates for finding terrestrial planets thanks to its brightness and position in the sky, which gives a long observational window each year from the southern hemisphere. However, detecting small, rocky planets the size of Earth is challenging because of the relatively small wobbles they induce in their host stars. Up to five years of dedicated observations may be needed to detect any around Alpha Centauri B.

“With tens of thousands of observations, we might hope to actually detect small rocky planets analogous to Earth and Mars,” says Debra Fischer, a team member at San Francisco State University. “That would be a huge breakthrough for understanding how frequently rocky planets form, and for understanding planet formation in binary star systems.”

The researchers have received some initial funding and will be starting an observational program in mid-May to monitor Alpha Centauri A and B intensively using the 1.5-meter telescope at the Cerro Tololo Inter-American Observatory in Chile. “In five years we could detect not only these Earths, but entire neighbouring planetary systems,” says Guedes.