So far almost 4,000 planets have been discovered orbiting stars other than the Sun, and that tally is constantly growing. Now another 18 exoplanets have been added to the haul, but the difference this time is that all of them are roughly Earth-sized – much smaller than the Neptune-sized average found so far. The discovery was made in old data, using a new and more sensitive algorithm.

Since Earth is the only planet known (so far) to harbor life, worlds like our own are of particular interest to exoplanet-hunters. That makes this new haul pretty exciting, especially given that these planets are on the small side of the scale and are usually harder to find than Jupiter-sized gas giants.

The smallest of the bunch is only about two-thirds the size of Earth, making it the second-smallest exoplanet detected to date. Two others are also smaller than Earth, while the rest of the crop range up to about twice the size of our home world. The most exciting new planet though is one that appears to be cool enough to potentially host liquid water on its surface, meaning it could be habitable.

These 18 new planets were discovered in old data by a team of German astronomers. Exoplanets are usually found using the transit method, which sounds simple enough in theory – essentially, telescopes like Kepler stare at stars and watch for sudden dips in their brightness. If that occurs on a regular schedule, it indicates that a planet is passing between the star and the telescope, blocking some of the light.

Obviously, larger planets make their presence known much more clearly, creating sharp drops in brightness. So it's no surprise that the majority of detected exoplanets are gas giants like Jupiter and Saturn, and ice giants like Uranus and Neptune.

"Standard search algorithms attempt to identify sudden drops in brightness," says René Heller, first author of two new studies. "In reality, however, a stellar disk appears slightly darker at the edge than in the center. When a planet moves in front of a star, it therefore initially blocks less starlight than at the mid-time of the transit. The maximum dimming of the star occurs in the center of the transit just before the star becomes gradually brighter again."

So the team developed a new, more sensitive method. It works on the same basic principle but rather than just looking for the abrupt changes, it takes into account the more gradual dimming and brightening that occurs when smaller planets pass in front of a star. They may not be as immediately obvious, but if they repeat regularly they're still a solid indication of a planet's existence.

To test out the new method, the researchers applied it to an old batch of data gathered during Kepler's second run, between 2014 and 2018. During that time the telescope identified 517 stars with at least one planet around them, and the team re-analyzed this group to see if they could find any smaller worlds that were missed the first time around. Lo and behold, 18 such planets emerged from the data.

Of course, this is just the beginning. There's plenty more data from Kepler and its planet-hunting successor TESS to trawl through in search of hidden Earth-like worlds.

Two separate studies were published in the journal Astronomy & Astrophysics [1], [2].

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