Space and Physics

NASA’s Planet-Hunting TESS Has Discovered Its Smallest Exoplanet Yet


Dr. Alfredo Carpineti

Senior Staff Writer & Space Correspondent

clockJun 28 2019, 17:04 UTC

The size of three exoplanets discovered in the L98-59 system by TESS compared to Mars and Earth. NASA’s Goddard Space Flight Center

NASA’s Transiting Exoplanet Survey Satellite, otherwise known as TESS, has discovered its smallest exoplanet yet. Known as L 98-59b, the planet is about 80 percent of Earth's size. Bigger than Mars but slightly smaller than Venus.


The world orbits an M-class star about one-third the mass of our Sun, and it has siblings. The two other planets, L 98-59c and L 98-59d, are respectively around 1.4 and 1.6 times bigger than Earth.  The discovery is reported in The Astronomical Journal.

Finding such a planet it is not an easy task, so it shows the incredible potential for TESS. It's estimated that over its 2-year primary mission, it's expected to find more than 20,000 exoplanets. Around 1,000 of them are expected to be Earth-sized – or smaller, clearly.  

L 98-59b is about 10 percent smaller than the previous record-holder that TESS has discovered. The smallest exoplanet ever observed is Kepler-37b, and it is only 20 percent larger than the Moon.

“The discovery is a great engineering and scientific accomplishment for TESS,” lead author Veselin Kostov, an astrophysicist at NASA’s Goddard Space Flight Center, said in a statement. “For atmospheric studies of small planets, you need short orbits around bright stars, but such planets are difficult to detect. This system has the potential for fascinating future studies.”


While the technology is almost there for these detailed observations, we can speculate about the nature of these planets. All three of them orbit their star very closely, receiving respectively 22, 11, and four times more radiant energy than Earth does. This puts them in the so-called Venus zone, the region in a planetary system where Earth-size planets might undergo a runaway greenhouse process and become hellish worlds like Venus.

Observatories like the James Webb Space Telescope (on plan for 2021) will be able to study these planets in more detail. So we could find out if the planets in the L 98-59 system are Earth-like, Venus-like, or completely different from our expectations.

"If we viewed the Sun from L 98-59, transits by Earth and Venus would lead us to think the planets are almost identical, but we know they’re not,” said Joshua Schlieder, an astrophysicist at Goddard, and co-author of the study. “We still have many questions about why Earth became habitable and Venus did not. If we can find and study similar examples around other stars, like L 98-59, we can potentially unlock some of those secrets.”

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