Nasa's newest planet-hunting telescope has found its first Earth-sized planet orbiting around two stars.

The world, known as TOI 1338 b, is in the habitable zone, which means that it orbits its suns at a distance that could allow it to support liquid water.

That, in turn, is thought to be a key ingredient for life, leading scientists to hope that the distant world could possible host aliens.

Nasa's TESS satellite was sent to space in the hope of spotting distant worlds as they move in front of their stars and block out the light. That makes it extra difficult to spot planets that orbit around two stars, making the discovery even more of a breakthrough.

The star system known as TOI 1338 is 1,300 light years away. It is made up of the newly discovered planet as well as its two suns, one of them slightly brighter than ours and the other cool and dim, which orbit around each other every 15 days.

The planet itself is almost seven times as big as Earth, or roughly somewhere between Neptune and Saturn.

The new TESS space telescope has four cameras on board, which are used to continually survey different patches of the sky. The aim is to spot when the stars it can see temporarily go dim, which could indicate that a planet has passed in front of them.

The dimming that would eventually be identified as TOI 1338 b was first spotted by an intern at Nasa named Wolf Cukier, who was looking through a series of candidates that had been uploaded to a citizen science project.

"I was looking through the data for everything the volunteers had flagged as an eclipsing binary, a system where two stars circle around each other and from our view eclipse each other every orbit," Cukier said.

"About three days into my internship, I saw a signal from a system called TOI 1338. At first I thought it was a stellar eclipse, but the timing was wrong. It turned out to be a planet."

Scientists have tried to use computers to spot such patterns, but they are made more difficult in binary star systems like TOI 1338. That meant that the work of those like Cukier was doubly important, to allow scientists to identify possible planets.

"These are the types of signals that algorithms really struggle with," said lead author Veselin Kostov, a research scientist at the SETI Institute and Goddard. "The human eye is extremely good at finding patterns in data, especially non-periodic patterns like those we see in transits from these systems."

Scientists have already examined the star system from the ground, which gave Kostov and his team historical data to use to understand the system and confirm the existence of the planet. While its orbit is stable, it changes in relation to us so that the dimming will not be visible for the eight years after November 2023.

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