The star closest to the Sun might have a second, hidden planet, scientists say.

Proxima Centauri, the solar system that is our nearest neighbour, is already known to be home to one planet.

But scientists say they have seen clues that there is another planet there, which remains mostly undiscovered.

The candidate planet orbits around the star every 5.2 years and is thought to be a "super-Earth". Its mass would be higher than our own planet, but not as large as the ice giants Uranus and Neptune that inhabit our own solar system.

If the possible planet is confirmed, it could show us how lower mass planets are formed around small stars, the scientists write in a new paper published in the journal Science Advances.

Nasa's groundbreaking decade of space exploration: In pictures Show all 10 1 /10 Nasa's groundbreaking decade of space exploration: In pictures Nasa's groundbreaking decade of space exploration: In pictures Mystic Mountain, a pillar of gas and dust standing at three-light-years tall, bursting with jets of gas from fledgling stars buried within, was captured by Nasa's Hubble Space Telescope in February 2010 Nasa/ESA/STScI Nasa's groundbreaking decade of space exploration: In pictures The first ever selfie taken on an alien planet, captured by Nasa's Curiosity Rover in the early days of its mission to explore Mars in 2012 Nasa/JPL-Caltech/MSSS Nasa's groundbreaking decade of space exploration: In pictures Death of a star: This image from Nasa's Chandra X-ray telescope shows the supernova of Tycho, a star in our Milky Way galaxy Nasa Nasa's groundbreaking decade of space exploration: In pictures Arrokoth, the most distant object ever explored, pictured here on 1 January 2019 by a camera on Nasa's New Horizons spaceraft at a distance of 4.1 billion miles from Earth Getty Nasa's groundbreaking decade of space exploration: In pictures An image of the Large Magellanic Cloud galaxy seen in infrared light by the Herschel Space Observatory in January 2012. Regions of space such as this are where new stars are born from a mixture of elements and cosmic dust Nasa Nasa's groundbreaking decade of space exploration: In pictures The first ever image of a black hole, captured by the Event Horizon telescope, as part of a global collaboration involving Nasa, and released on 10 April 2019. The image reveals the black hole at the centre of Messier 87, a massive galaxy in the nearby Virgo galaxy cluster. This black hole resides about 54 million light-years from Earth Getty Nasa's groundbreaking decade of space exploration: In pictures Pluto, as pictured by Nasa's New Horizons spacecraft as it flew over the dwarf planet for the first time ever in July 2015 Nasa/APL/SwRI Nasa's groundbreaking decade of space exploration: In pictures A coronal mass ejection as seen by the Chandra Observatory in 2019. This is the first time that Chandra has detected this phenomenon from a star other than the Sun Nasa Nasa's groundbreaking decade of space exploration: In pictures Dark, narrow, 100 meter-long streaks running downhill on the surface Mars were believed to be evidence of contemporary flowing water. It has since been suggested that they may instead be formed by flowing sand Nasa/JPL/University of Arizona Nasa's groundbreaking decade of space exploration: In pictures Morning Aurora: Nasa astronaut Scott Kelly captured this photograph of the green lights of the aurora from the International Space Station in October 2015 Nasa/Scott Kelly

It could also lead us to re-think our understanding of how such super-Earths are formed. Scientists believe that they largely come about in the "snowline", where they are at the right distance from their star as to have water turn into solid ice, but the planet would dwell much further away than that zone.

The other known planet that inhabits the same solar system, which is known as Proxima b, already thrilled astronomers when it was discovered. It is described as perhaps one of the best places anywhere for humanity to potentially find life, with all the right conditions, and is close enough that it could one day be visited by robots from Earth.

Scientists have long wondered whether Proxima Centauri could have another planet orbiting around it.

That became even more of a possibility when the Atacama Large Millimeter/submillimeter Array observatory in Chile spotted an unknown source of light spectrum signals coming from the solar system. At the time, scientists wondered whether the mysterious light was coming from another planet, leaking out from a nearby galaxy – or if it might be something else entirely.