NASA's planet-hunting telescope found an Earth-size exoplanet 100 light-years away. It could hide a vast ocean.

• Astronomers have discovered a new Earth-sized planet outside our solar system, named TOI 700 d, 100 light-years away.
• The planet is in its star's habitable zone and has the potential to hold liquid water.
• Future research could give scientists more information about the planet's surface and atmosphere.
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NASA astronomers have found yet another planet outside our solar system that has the potential to host alien life.

On Monday, a team of researchers announced that the agency's planet-hunting telescope, the Transiting Exoplanet Survey Satellite (TESS), has discovered an Earth-sized exoplanet that has the potential to hold liquid water.

The planet, called TOI 700 d, is about 100 light-years away, and it orbits a red dwarf star with about 40% the mass of our sun.

TOI 700 d sits in its star's Goldilocks-like habitable zone - the area around a star where it's not too hot and not too cold for water on orbiting planets to remain in a liquid state. This particular exoplanet (the term for planets outside our solar system) is about 20% bigger than Earth, and it receives about 86% as much light energy as Earth gets from the sun. It's one of only five Earth-sized planets researchers have ever found in a star's habitable zone.

It's also the first Earth-sized planet TESS has ever found within its star's habitable zone. (The space telescope became operational in 2018.)

Exoplanets that orbit red dwarfs like TOI 700 aren't always the best places to look for life, since such stars are prone to powerful flares. Those can fry a planet's atmosphere, making the chance of alien life there very slim.

But in the case of TOI 700, the conditions seem just right.

"In 11 months of data, we saw no flares from the star, which improves the chances TOI 700 d is habitable and makes it easier to model its atmospheric and surface conditions," Emily Gilbert, who leads the team that found the exoplanet, said in a NASA press release.

Three planets, one star

TESS hunts for planets using what's known as the transit method: The telescope measures light from thousands of stars, then notes when a particular star's light dims, since that's a sign that a planet has passed in front of it.

This discovery, announced at the annual meeting of the American Astronomical Society, showed that two other planets orbit the same star as TOI 700 d.

The innermost one of the trio is a rocky world that's also close to the size of Earth, but it's far too close to its star to be habitable. The middle planet is likely a miniature version of Neptune - a gaseous world 2.6 times as big as Earth - but it's not in the habitable zone, either.

All three planets are tidally locked, meaning the same side of the planet faces the star at all times, leaving the other half in perpetual darkness.

In order to be sure of their finding , a second telescope - NASA's Spitzer Space Telescope - also observed the star system to confirm TESS's findings.

Modeling TOI 700 d's atmosphere

The fact that TOI 700 d is the same size as our planet doesn't mean that conditions on its surface mirror Earth's.

Until astronomers can figure out how much mass TOI 700 d has, they can't be sure if the planet is a rocky world like ours or gaseous one like one of its sisters.

Upcoming, advanced telescopes, however, could enable scientists to make those measurements. NASA's James A. Webb telescope, which is slated to come online in 2021, could collect the data astronomers need to get a clear picture of what TOI 700 d's surface is like. Plus, because TOI 700 is a bright star relatively close to Earth, ground-based telescopes could also look to see how much its starlight pierces the three exoplanets' atmospheres.

For now, though, a second research team offered several models of what the TOI 700 d planet's environment might look like, based on its size and the type of star it orbits.

One of those models suggested the exoplanet could be covered in a vast ocean, with a carbon-dioxide-dominated atmosphere similar to what scientists suspect surrounded Mars when it was young. Another model proposed TOI 700 d could be a cloudless version of our modern Earth that's all land with no sea.

"It's exciting because no matter what we find out about the planet, it's going to look completely different from what we have here on Earth," Gabrielle Engelmann-Suissa, a research assistant at NASA's Goddard Space Flight Center, said in a press release.

Other exoplanets are even more promising

TOI 700 d is not the only planet in the universe that researchers think could be habitable for alien life. In fact, it's far from the front-runner.

In September 2019, scientists announced that they'd detected water vapor on a potentially habitable planet for the first time. The planet, called K2-18b, is a super-Earth that orbits a red dwarf star 110 light-years away.

K2-18b is the only known exoplanet with water vapor, an atmosphere, and a temperature range that could support liquid water on its surface. That makes it our "best candidate for habitability," one researcher said.

In 2016, astronomers also discovered three Earth-size planets similar to TOI 700 d orbiting a dim red dwarf star about 39 light-years away. A year later, they discovered that seven planets actually pass through the habitable zone of that star, named TRAPPIST-1. Three or four of those planets may have conditions conducive to extraterrestrial life.

"We've made a giant, accelerated leap forward in the search for habitable worlds," Sara Seager, a planetary scientist at MIT who wasn't involved in the research, said of the TRAPPIST-1 exoplanets in February 2017. "In this system, it's like Goldilocks has many sisters."