Scientists have characterised the first potentially habitable world outside our own solar system located about 31 light-years away. The ‘super-Earth planet’ — named GJ 357 d — was discovered in early 2019 owing to NASA's Transiting Exoplanet Survey Satellite (TESS), a mission designed to comb the heavens for exoplanets, according to the research published in the Astrophysical Journal Letters.

"This is exciting, as this is humanity's first nearby super-Earth that could harbour life — uncovered with help from TESS, our small, mighty mission with a huge reach," said Lisa Kaltenegger, associate professor of astronomy at Cornell University in the US and a member of the TESS science team.

The exoplanet is more massive than our own blue planet, and Kaltenegger said the discovery will provide insight into Earth's heavyweight planetary cousins.

"With a thick atmosphere, the planet GJ 357 d could maintain liquid water on its surface like Earth, and we could pick out signs of life with telescopes that will soon be online," she said.

Astronomers from the Institute of Astrophysics of the Canary Islands and the University of La Laguna, both in Spain, announced the discovery of the GJ 357 system in the journal Astronomy & Astrophysics. They showed that the distant solar system — with a diminutive M-type dwarf sun, about one-third the size of our own sun — harbours three planets, with one of those in that system's habitable zone: GJ 357 d.

Last February, the TESS satellite observed that the dwarf sun GJ 357 dimmed very slightly every 3.9 days, evidence of a transiting planet moving across the star's face.

That planet was GJ 357 b, a so-called "hot Earth" about 22 per cent larger than Earth, according to the NASA Goddard Space Flight Center, which guides TESS. Follow-up observations from the ground led to the discovery of two more exoplanetary siblings: GJ 357 c and GJ 357 d.

The international team of scientists collected Earth-based telescopic data going back two decades -- to reveal the newly found exoplanets' tiny gravitational tugs on its host star, according to NASA.

Exoplanet GJ 357 c sizzles at 127 degrees Celsius and has at least 3.4 times Earth's mass.

However, the system's outermost known sibling planet — GJ 357 d, a super-Earth -- could provide Earth-like conditions and orbits the dwarf star every 55.7 days at a distance about one-fifth of Earth's distance from the sun. It is not yet known if this planet transits its sun.

Kaltenegger, doctoral candidate Jack Madden and undergraduate student Zifan Lin simulated light fingerprints, climates and remotely detectable spectra for a planet that could range from a rocky composition to a water world.

"We built the first models of what this new world could be like. Just knowing that liquid water can exist on the surface of this planet motivates scientists to find ways of detecting signs of life," Madden said.

"If GJ 357 d were to show signs of life, it would be at the top of everyone's travel list — and we could answer a 1,000-year-old question on whether we are alone in the cosmos," Kaltenegger said.