The newly-discovered planet has a radius of 1.03 Earth radii and a temperature between 27 and 127 degrees Celsius (81-261 degrees Fahrenheit), with the permanent night side plausibly allowing for water condensation. Named LP 791-18d, this so-called exo-Earth could undergo volcanic outbursts as often as Jupiter’s moon Io, the most volcanically active body in the Solar System.
An artist’s impression of the exo-Earth LP 791-18d. Image credit: NASA’s Goddard Space Flight Center / Chris Smith, KRBwyle.
The newfound planet orbits LP 791-18, a small red dwarf star about 86 light-years away in the southern constellation of Crater.
Also known as TOI-736 or TIC 181804752, the star is much smaller and less bright than our Sun.
Astronomers already knew about two other worlds in the system: LP 791-18b and c.
The inner planet, LP 791-18b, is about 20% bigger than Earth, and circles the star in a little less than a day.
The outer planet c is about 2.5 times Earth’s size, more than 7 times its mass, and has a period of about 5 days.
The new planet, LP 791-18d, has a radius of 1.03 Earth radii and a mass of 0.9 Earth masses.
It has an orbital period of only 2.8 days and sits on the inner edge of the habitable zone, the traditional range of distances from a star where scientists hypothesize liquid water could exist on a planet’s surface.
During each orbit, planets LP 791-18d and c pass very close to each other.
Each close pass by the more massive LP 791-18c produces a gravitational tug on LP 791-18d, making its orbit somewhat elliptical.
On this elliptical path, LP 791-18d is slightly deformed every time it goes around the star.
These deformations can create enough internal friction to substantially heat the planet’s interior and produce volcanic activity at its surface. Jupiter and some of its moons affect Io in a similar way.
“LP 791-18d is tidally locked, which means the same side constantly faces its star,” said Université de Montréal’s Professor Björn Benneke.
“The day side would probably be too hot for liquid water to exist on the surface. But the amount of volcanic activity we suspect occurs all over the planet could sustain an atmosphere, which may allow water to condense on the night side.”
Professor Benneke and colleagues discovered LP 791-18d using data from NASA’s TESS telescope and retired Spitzer Space Telescope, as well as a suite of ground-based observatories.
“The discovery of an Earth-size planet in the habitable zone that is potentially volcanically active is a huge step forward in the search for life outside the Solar System,” said Dr. Mohamad Ali-Dib, an astronomer at the Université de Montréal and New York University Abu Dhabi.
“The next step is to observe the system using Webb to see what it can tell us about its atmosphere.”
The discovery is described in a paper in the journal Nature.
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M.S. Peterson et al. A temperate Earth-sized planet with tidal heating transiting an M6 star. Nature, published online May 17, 2023; doi: 10.1038/s41586-023-05934-8
