It’s hard to remember that there was a time in the not-so-distant past when we didn’t know if planets existed outside our solar system. Scientists were pretty sure other stars harbored worlds, but we had no idea how to find them. Then, in 1992, NASA found the first one, and we’ve been searching for Earth-like planets ever since.

To date, NASA has confirmed the existence of 5,000 exoplanets, but we know at this point our galaxy probably harbors way more than that — likely in the hundreds of billions. But the question is how many Earths are there? NASA scientists have been hard at work searching for Earth-like planets outside our solar system and have even found a few. Here’s what scientists are looking for, how we find them, and why it matters.

What Makes Planets Similar to Earth?

It’s easy to talk about “Earth-like” planets, but what does that mean exactly? Let’s break it down. Earth is in what’s called the “habitable zone” of our star — it’s close enough to our star that water doesn’t freeze on the surface, but it’s far enough away to where it doesn’t boil off. It’s the perfect distance from our star for liquid water to exist on the planet’s surface.

The complicated thing, though, is that we can’t just look for planets that are the same distance away from their stars as the Earth is from the sun. The temperature of the star is an important factor. If the star is cooler than the sun — a red giant, for example — the planet needs to be much closer to its star to be in the habitable zone.

Astronomers also look for planets with a similar composition to Earth — ones that are rocky. We’ve found numerous gas giants like Jupiter and ice giants like Neptune, but finding a rocky planet within the habitable zone of its star is key. The good news? We’ve found plenty of them.

How Do We Find Earth-Like Exoplanets?

But how do we do that? How do researchers actually discover exoplanets?

NASA has a planet-hunting spacecraft, the Transiting Exoplanet Survey Satellite (TESS). Before TESS was the Kepler spacecraft, which discovered many exoplanets until it ran out of fuel in 2018.

TESS works by surveying the brightest 200,000 stars in our sky. It points its camera at an area of the sky for about a month, looking for any variations in a star’s brightness.

Despite how stars may look in our night sky from the ground, they don’t actually twinkle in space. Their brightness is steady. That “twinkling” effect is a result of the Earth’s atmosphere. Therefore, if TESS detects changes in a star’s brightness, it’s possible that’s because of transiting — a planet passing in front of a star, as seen from Earth, blocking a tiny amount of its light.

When TESS detects a possible transit, it catalogs the star. Scientists then review the data to confirm or debunk the existence of an exoplanet. (It’s important to note there are other reasons a star’s brightness could change — it could be entering a different phase of its life or could just expel dust that temporarily dims it from our perspective).

How Many Earths Are There?

The question scientists have been asking for years is how many Earth-like planets could there actually be? After all, it seemed like we were only finding large, Jupiter-like planets. But there’s a good reason for that: These planets are much bigger and simply easier to detect than smaller rocky planets.

The fact is astronomers think there could be billions of Earths out there. Natalie Batalha, a mission scientist on the Kepler project and astronomer at the University of California, says that — thanks to the Kepler spacecraft — we know that most stars have planets. We also know that there are actually more inner, rocky planets than there are large planets out there.

“We have learned that every star has at least one planet, and that there are tens of billions of potentially habitable, Earth-size planets,” Dr. Batalha said in the NASA release. Could that mean one of them might harbor life? It’s certainly possible.

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