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Data: PHL’s Exoplanets Catalog. Note: Earth Similarity Index was introduced by Schulze-Makuch et al., Astrobiology, 2011. ESI is calculated with respect to a planet’s size and the energy received by its star; it is not necessarily a corollary for potential habitability. Chart: Harry Stevens/Axios

Astronomers have confirmed the existence of more than 3,000 "exoplanets" — planets that aren't in our solar system, orbiting stars other than our sun. The chart above shows how similar they are to Earth.

Why it matters: A small number of discovered exoplanets — those that are small enough to have a rocky surface and the right distance from their star to hold liquid water — may be able to support life, and provide clues about the evolution of our own planet.

The big picture: The first discovery of a planet orbiting a star other than the sun was only 30 years ago.

  • In 1988, scientists announced the discovery of a planet orbiting a pulsar nearly 5,000 light-years away.
  • Over the last three decades, the pace of discovery has only quickened: More than half of confirmed exoplanet discoveries have been announced in the last four years.

Most of the recent discoveries were detected by the Kepler space telescope, the first space-based telescope designed specifically to find Earth-sized planets orbiting other stars. "Kepler was a complete game-changer," MIT astronomer Sara Seager told Axios.

  • That's why Seager is enthusiastic about analyzing the findings from Kepler's successor, the Transiting Exoplanet Survey Satellite (TESS), which was launched in April.
  • TESS searches for Earth-like planets in an area of the sky closer to Earth than where Kepler looked, making it easier for astronomers to later confirm the planets' existence by calculating their mass.

Since TESS began making observations in July, it has already made two confirmed exoplanet discoveries. Scientists expect it to make thousands more as it sends its findings back to Earth. "Every month we get a deluge of data," Seager said.

What's next: Neither TESS, nor Kepler, nor any of the massive land-based telescopes searching the galaxy for planets are able to capture actual images of exoplanets.

  • Instead, they detect their presence by measuring changes in their stars' light over time.

The bottom line: The technology for capturing even pixel-sized images of exoplanets — which would help scientists better assess whether their atmospheres are suitable for life — is still many years away.

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