The closest star to the Sun is Proxima Centauri, a red dwarf more than 4 light-years away. A few years ago researchers discovered that an Earth-sized world orbited the nearby star and now there seem to be hints of a second planet.

A new paper in Science Advances presents the data and makes a case for the candidate planet. The Atacama Large Millimeter/submillimeter Array (ALMA) has previously detected an unknown source of emission from the area around Proxima Centauri and for this reason, this team decided to take a good look at the star.

They had 17.5 years’ worth of data and employed a technique called the radial-velocity method. They looked at how much the star wobbles and discerned whether this is caused by the internal processes of the star or a nearby planet tugging at it.

The data suggest a periodic wobble and the team considers the most likely explanation a planet weighing at least 5.8 times the mass of Earth, orbiting around the star every 5.2 years. Given the length of this orbital period, the team doesn’t think the wobble is related to cycles in the star itself, but rather the planet.

“Proxima Centauri is the nearest star to the Sun and this detection would make it the closest planetary system to us,” lead author Dr Mario Damasso of Italy's National Institute for Astrophysics told IFLScience. “In my opinion, the detection is particularly interesting because the signal in the radial velocities has a low amplitude and a long period, therefore in principle it's not easy to detect and confirm with a single technique.”

The researchers are very cautious to state that while the finding is exciting, it is not a discovery just yet. Nevertheless, the team is already at work to strengthen the case for the planet's existence. More analyses will soon be published and they even hope to try to image the planet directly, a very difficult feat.

But the crucial data to confirm or deny the existence of this candidate planet will come from the European Space Agency’s Gaia satellite. Gaia is measuring the detailed position and motion of 1.8 billion stars and its precise eye is invaluable to study objects near and far.

“If the planet is there, our study shows that Gaia should detect it easily. We have to be patient and need to wait a few years more for the final public release of Gaia data," said Damasso. "However, Proxima needs to be followed up with spectrographs, increasing the number of radial velocity data over the next years, also using next-generation instruments like ESPRESSO, that should provide very high precision measurements.”

If the planet exists it will be called Proxima c, but the team is aware that the task ahead of them is likely to be a difficult one. After all, they're armed with nothing more than the star's wobbles.