(Nanowerk News) A large international team led by astronomers at the Trottier Institute for Research on Exoplanets at the Université de Montréal (UdeM) announced in the journal Natural (“A moderately Earth-sized planet with tidal heating transiting star M6”) the discovery of a new temperate world around a nearby small star.
The planet, named LP 791-18 d, has the same radius and mass as Earth. Observations of this exoplanet and others in the same system suggest that LP 791-18 d is likely covered by volcanoes similar to Jupiter’s moon Io, the most volcanically active body in our Solar System.
“The discovery of this exoplanet is an extraordinary discovery,” said Professor Björn Benneke from UdeM’s Department of Physics. “The shared properties of LP 791-18 d and Earth and the prospects for detectable geological activity and volcanism on it make it a key object for better understanding how terrestrial worlds formed and evolved.”
Thanks to the star’s diminutive size – which is only slightly larger than the planet Jupiter – this exoplanet’s atmosphere should be detectable, if any, using the James Webb Space Telescope.
A new terrestrial world in a multi-planetary system
The planet discovery was led by Merrin Peterson, a graduate student on Benneke’s team at the Trottier Institute. It was found and studied using data from NASA’s Spitzer Space Telescope and the Transiting Exoplanet Survey Satellite (TESS), as well as a series of ground-based observatories around the world.
LP 791-18 d orbits a small red dwarf about 86 light years away in the constellation South Crater. The Spitzer telescope noticed the star’s infrared light dimmed slightly as the planet passed in front of its disk, a phenomenon called a transit. Observations of the system in October 2019 were among the last Spitzer rounds before being decommissioned.
Benneke’s team used the data to determine that the planet is about the same size as Earth. With an orbital period of only 2.8 days, exoplanet LP 791-18 d is very close to its parent star. However, the star is much smaller and less bright than our sun. The temperature on LP-791-18 d is thus only slightly higher than on Earth.
Astronomers have known about the other two worlds in this system, planets LP 791-18 b and c, since 2019, when they were detected by the TESS satellite. Planet b is about 20% larger than Earth and circles its star in less than a day, while Planet c is about 2.5 times the size of Earth and has a period of about 5 days. The newly discovered exoplanet d is the smallest in the family and orbits between planets b and c at an intermediate distance from its star.
Intense volcanic activity
By tracking the planets’ movements precisely, the team realized that planets c and d passed very close to each other as they traced their orbits. At their closest, they are only 1.5 million kilometers away, which is 33 times closer than Mars and Earth are from each other. Any closer distance between the planets produces a gravitational pull on planet d, making its orbit less circular and more elliptical. Along this elliptical path, planet d changes shape slightly each time it circles the star. Astronomers calculate that this deformation generates a lot of heat in the planet’s interior that needs to be transported to the surface through intense volcanic activity. Due to this phenomenon called tidal heating, LP 791-18 d is likely covered with volcanoes.
“Significant friction generated by tidal heating of the planet is responsible for heating its interior to some extent, which in turn allows for subsurface oceans of magma,” explains Caroline Piaulet, a UdeM Ph.D. students involved in the discovery. “In our Solar System, we know that Jupiter’s moon Io is influenced by Jupiter and other moons in the same way, and that world is the most volcanic we know of.”
Planet d is on the inner edge of the temperate (or “habitable”) zone, the traditional distance from the star where scientists hypothesize liquid water could exist on the planet’s surface. If the planet is as geologically active as the UdeM team suspects, it could retain an atmosphere. Given the right conditions, temperatures can even drop low enough on the night side of the planet for water to condense on the surface.
Measure the mass of the planet
The proximity of Planets c and d also helps scientists measure their masses. When approached by Benneke with this discovery, many members of TESS’ Follow-up Observation Program switched their ground-based telescopes to the LP 791-18 system, which allowed them to collect observations of 67 transits of Planets c and d.
In this data, the team at UdeM were able to detect variations in transit times, which are small differences in the exact times of planets’ transits caused by the gravitational pull they exert on one another.
“This technique allows us to estimate the masses of Planets c and d using only transit data,” explained Piaulet. “By combining this information with planet size – information available from the same data – we can estimate planet densities and thus get an idea of their composition and properties.”
Comparing these values with models of the planet’s interior, astronomers were thus able to determine that the newly discovered planet has a mass comparable to that of Earth. Its density is also consistent with the composition of rocks like Earth. Planet c, which has a mass about 7 times that of Earth, may have deposited large amounts of gas or light material, similar in composition to Neptune.
‘It’s important to analyze the atmosphere’
Planet c, the largest system, has been approved for observing time at the Webb Telescope, as part of Canada’s NEAT program, dedicated to the study of exoplanets. Pierre-Alexis Roy, Ph.D. students on Benneke’s team at UdeM, will be tasked with analyzing these observations. “Having precise limits on the mass of Planet c will be critical to analyzing the substantial atmosphere we expect to find on this mini-Neptune,” he explained.
In the future, the small size of the star will allow even the detection of a much wider atmosphere on the newly discovered planet d. Scientists suspect that an atmosphere like that of Earth, Venus, or Saturn’s moon Titan could exist on Planet d. These systems represent an unparalleled opportunity to learn more about small rocky planets, such as the TRAPPIST-1 system, which hosts seven Earth-sized planets, and has been carefully studied by Webb. Thus, this system became Webb’s main target in the future. several years representing a similar opportunity with the TRAPPIST-1 system, which hosts seven Earth-sized planets.
“This system provides astronomers with a valuable laboratory to test various hypotheses related to the formation and evolution of terrestrial planets,” said Benneke. “The newly discovered planet d, a possibly volcano-enclosed Earth-sized world in a multiplanetary system, provides unprecedented opportunities to advance not only astronomy but many other fields of science, particularly geology, planetary science, atmospheric science, and possibly astrobiology. ”