Astronomers believe they have found the first potentially habitable planet near to our solar system – and they predict that signs of life, if there are any, will soon be detectable by the new generation of telescopes coming online.
The new world circles a dwarf star named GJ 357 discovered in July last year, and located 31 light-years from Earth in the constellation Hydra.
The M-type dwarf has about a third of our sun’s mass and size and is about 40 per cent cooler – and, in more recent news, has at least three planets in its orbit. One sits in the star’s habitable zone.
The way the discoveries played out is somewhat intriguing.
In February, cameras aboard NASA’s exoplanet-seeking space telescope – the Transiting Exoplanet Survey Satellite (TESS) – caught the dwarf star dimming slightly every 3.9 days, revealing the presence of a transiting exoplanet that passes across the face of the star during every orbit and briefly dims the star’s light.
That planet was GJ 357 b, and is what’s known as a “hot Earth” – being 22 per cent larger than Earth, and baking constantly about 127 degrees Celsius, according to the NASA Goddard Space Flight Centre.
To confirm the presence of GJ 357 b, and subsequently discover its neighbours, scientists investigated existing ground-based measurements of the star’s radial velocity, or the speed of its motion along our line of sight.
An orbiting planet produces a gravitational tug on its star, which results in a small reflex motion that astronomers can detect through tiny colour changes in the starlight.
Rafael Luque, a doctoral student at the Institute of Astrophysics of the Canary Islands (IAC) on Tenerife who led the discovery team, examined measurements stretching back to 1998 – and there they confirmed three planets were orbiting dwarf star GJ 357.
“In a way, these planets were hiding in measurements made at numerous observatories over many years,” Mr Luque said.
“It took TESS to point us to an interesting star where we could uncover them.”
GJ 357 c, the middle planet, was found to have a mass at least 3.4 times Earth’s, and orbits the star every 9.1 days – at more than twice the distance from the star as that of the transiting, closer planet.
Much further out is GJ 357 d, a super-Earth planet in the star’s habitable zone.
The planet weighs at least 6.1 times Earth’s mass, and orbits the star every 55.7 days at a range about 20 per cent of Earth’s distance from the Sun.
The planet’s size and composition are unknown, but a rocky world with this mass would range from about one to two times Earth’s size.
“GJ 357 d is located within the outer edge of its star’s habitable zone, where it receives about the same amount of stellar energy from its star as Mars does from the Sun,” said co-author Diana Kossakowski at the Max Planck Institute for Astronomy in Heidelberg, Germany.
“If the planet has a dense atmosphere, which will take future studies to determine, it could trap enough heat to warm the planet and allow liquid water on its surface.”
The international team of astronomers was led by Associate Professor Lisa Kaltenegger, director of the Cornell University’s Carl Sagan Institute.
In a prepared statement from the university, she said: “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.
“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.”
A study of the team’s findings was published this week in Astronomy & Astrophysics.