The Transiting Exoplanet Survey Satellite, or TESS, which has been searching for planets like Earth outside of the solar system, unexpectedly went into "safe mode" Monday, NASA said.

The space agency said it is investigating the cause of TESS's shutdown, which happened during routine activities. The satellite remains in good condition, and NASA expects to have it back online "in the coming days."

SpaceX launched TESS in 2018 to replace the decommissioned Kepler telescope as NASA's primary planet-hunter. The satellite surveys bright stars for evidence of planetary objects in their orbit with a wider and more comprehensive view than its predecessor.

TESS in 2021 captured a bright gamma-ray burst, which indicates the collapse of a massive star and the birth of a black hole, proving the satellite is useful for observing other interstellar phenomena.

The satellite in 2022 discovered two rocky, Earth-like exoplanets some 33 light-years away from Earth, the closest rocky exoplanets ever found.

Both planets, dubbed "super earths" for being about 1 1/2 times the size of Earth, orbit the red dwarf star HD 260655. NASA scientists said the planets might be too hot to sustain life.

Researchers from Berkeley's SETI Research Center said in February they utilized TESS data to help identify new possible signals of intelligent life.

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