In the hunt for habitable worlds outside the Solar System, this may be a bit of a breakthrough. Astronomers have found four possible planets around the nearest Sun-like star to Earth, and two of them could be habitable.

The discovery was led by Fabo Feng from the University of Hertfordshire, and has been accepted for publication in the Astronomical Journal. A pre-print is available on arXiv.

The star is Tau Ceti, located 12 light-years away. It has about 78 percent of the Sun’s mass, and is thought to be about 5.8 billion years old.

So far, we’ve found a few planets in our vicinity but all have been around dim, red dwarf stars. That Tau Ceti is more like our Sun is pretty exciting.

The researchers used the radial velocity method, which notices gravitational tugs on a star, to find the planets. They are thought to be in orbits of 20 days (Tau Ceti g), 49.3 days (h), 160 days (e), and 600 days (f). The former two have masses of at least 1.7 Earths, and the latter 3.9, making them super-Earths.

"We are quite confident about the discoveries," Feng told IFLScience. "We have also found a signal with a period of 90 days although it is not significant enough to be confirmed as a planet."

Tau Ceti e and f are of particular interest, as they orbit in or near the star's habitable zone. The former orbits on the inner edge, where liquid water could exist. The latter orbits on the outer edge.

Unfortunately, life on these worlds may not be possible. The astronomers found evidence for a huge disk of debris in orbit around the star, which may cause asteroids and comets to hit the planets.

"Their habitability might be strongly reduced by the bombardments of objects from the massive scattered disc," the team notes in their paper. They also said that if the disk and the planets are orbiting in the same plane, then the masses of the planets might be double.

The existence of the planets is also not certain. This detection involved using the HARPS spectrograph on the Keck observatory in Hawaii to see tiny perturbations in the star. These were as small as 30 centimeters per second. However, it cannot be certain yet these perturbations were from planets; Tau Ceti was previously theorized to have planets in 2013.

Interestingly, this precision is important for finding smaller and smaller exoplanets via the radial velocity method. If we can get it down to 10 centimeters a second, we could find planets more similar to Earth in size.

"Our detection of such weak wobbles is a milestone in the search for Earth analogs,” Feng said in a statement.