Hot Exoplanet Smaller than Earth Found Orbiting Nearby Red Dwarf

Hot Exoplanet Smaller than Earth Found Orbiting Nearby Red Dwarf

An international team of astronomers has discovered a hot terrestrial planet orbiting the rapidly-rotating low-mass star TOI-540.

An artist’s impression of the hot rocky exoplanet TOI-540b and its parent star. Image credit:

TOI-540 is an M-type dwarf star located 45.7 light-years away in the constellation of Pictor.

Otherwise known as 2MASS J05051443-4756154 and UCAC4 211-005570, the star is only 16% the mass of the Sun, 19% of its radius, and is between 100 million and two billion years old.

TOI-540 is highly magnetically active, and has a flare rate that is consistent with other stars of a similar mass and rotation period.

“With a rotation period of 17.4 hours, TOI-540 is rotating more rapidly than TRAPPIST-1, the only other rapidly-rotating planet host within 49 light-years,” said lead author Kristo Ment from the Harvard & Smithsonian Center for Astrophysics and his colleagues from Canada, Chile, the United Kingdom, and the United States.

The newly-discovered planet, dubbed TOI-540b, completes a trip around its parent star once every 1.24 days.

It has a radius of 0.9 Earth radii, slightly less than that of Venus, an estimated mass of 0.7 Earth masses and a density of 5.2 g/cm3.

The planet is likely to be hot — it receives 23.4 times the total radiation from its host star than Earth does from the Sun, and 3.5 times more than Mercury does, equating to a surface temperature of 338 degrees Celsius (640 degrees Fahrenheit).

TOI-540b was discovered in photometric data from NASA’s Transiting Exoplanet Survey Satellite (TESS).

Follow-up observations with the MEarth-South telescope array at the Cerro Tololo International Observatory in Chile confirmed the transits of the planet.

“TOI-540b is a prime target for transmission spectroscopy to study a potential high mean molecular weight atmosphere, if indeed such an atmosphere can be retained in close proximity to an active M-dwarf,” the astronomers said.

A paper on the discovery will be published in the Astronomical Journal.


Kristo Ment et al. 2020. TOI 540 b: A Planet Smaller than Earth Orbiting a Nearby Rapidly Rotating Low-mass Star. AJ, in press; arXiv: 2009.13623

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