LP 791-18 d, illustrated here, is an Earth-size world about 86 light-years away. The gravitational tug from a more massive planet in the system, shown as a blue dot in the background, may result in internal heating and volcanic eruptions on this exoplanet – as much as Jupiter’s moon Io, the most geologically active body in the Solar System.
A sizable global group, headed by astronomers at the Trottier Institute for Research on Exoplanets at Université de Montréal (UdeM), revealed today in the journal Nature the detection of a fresh moderate planet encircling a close neighboring star.
This celestial body, designated LP 791-18 d, exhibits dimensions and weight akin to Earth's. Data from observations of LP 791-18 d and a companion exoplanet within the identical system propose that LP 791-18 d is probably adorned with volcanic formations resembling those found on Io, Jupiter's moon and the most eruptively dynamic entity within our Solar System.
"The detection of this exoplanet is a remarkable revelation," expressed Professor Björn Benneke from UdeM's Department of Physics. "The resemblances between LP 791-18 d and Earth in terms of their characteristics, coupled with the potential for observable geological activity and volcanism, render it a pivotal entity for enhancing our comprehension of the formation and progression of terrestrial planets."
Due to the minuscule magnitude of the star, marginally larger than Jupiter, there exists a promising prospect of discerning the atmosphere of this exoplanet, should it possess one, employing the James Webb Space Telescope.
A new terrestrial world in a multi-planetary system
Merrin Peterson, a graduate student in Benneke's team at the Trottier Institute, spearheaded the detection of the planet. The investigation involved the utilization of data obtained from NASA's Spitzer Space Telescope and Transiting Exoplanet Survey Satellite (TESS), along with a collection of terrestrial observatories dispersed globally.
Located approximately 86 light-years away in the southern constellation Crater, LP 791-18 d revolves around a diminutive red dwarf star. The Spitzer Telescope captured a slight reduction in the star's infrared luminosity when the planet transited across its disk, a phenomenon commonly referred to as a transit. The October 2019 observations of this system were among the final data gathered by Spitzer prior to its decommissioning.
Utilizing the collected data, Benneke's team concluded that LP 791-18 d bears a striking resemblance in size to Earth. Positioned in close proximity to its host star, the exoplanet boasts a mere 2.8-day orbital period. Nonetheless, its host star is considerably smaller and less radiant than our Sun. Consequently, LP 791-18 d exhibits a temperature only slightly higher than that of Earth.
Astronomers have been aware of two other planets within this system, namely LP 791-18 b and c, since their detection by the TESS satellite in 2019. Planet b surpasses Earth's size by approximately 20% and completes an orbit around its star in slightly less than a day. On the other hand, planet c boasts a size 2.5 times larger than that of Earth and has a period of approximately 5 days. The recently discovered exoplanet, d, assumes the smallest size among the family and resides in an intermediate distance between planets b and c, encircling the star.
Intense volcanic activity
Through meticulous tracking of the planet's motion, the research team discovered that planets c and d have extremely close encounters as they follow their respective orbits. At their closest approach, the distance between them is a mere 1.5 million kilometers, which is approximately 33 times closer than the minimum separation between Mars and Earth. These close encounters result in a gravitational pull on planet d, causing its orbit to become less circular and more elliptical. Along this elliptical trajectory, planet d undergoes slight deformations during each revolution around the star. The scientists calculated that this deformation generates substantial heat within the planet's interior, necessitating the transfer of this heat to the surface through vigorous volcanic activity. This process, known as tidal heating, suggests that LP 791-18 d is likely adorned with a multitude of volcanoes.
"The substantial friction caused by tidal heating within the planet plays a vital role in heating its interior to a significant degree, thereby facilitating the presence of a subsurface magma ocean," elucidated Caroline Piaulet, a Ph.D. student from UdeM who participated in the discovery. "In our own Solar System, we have observed a similar phenomenon with Jupiter's moon Io, which is influenced by Jupiter and its other moons, resulting in its status as the most volcanically active body known to us."
Positioned at the inner edge of the temperate, or "habitable," zone, planet d occupies the conventional range of distances from its star where scientists postulate the possibility of liquid water existing on a planet's surface. Should the planet exhibit the anticipated level of geological activity, it has the potential to retain an atmosphere. Under favorable circumstances, temperatures could even decrease sufficiently on the planet's nighttime side, leading to the condensation of water on its surface.
Measuring the masses of the planets
The close proximity of planets c and d has proven advantageous in enabling scientists to determine their respective masses. Upon sharing this discovery with Benneke, numerous participants of the TESS Follow-up Observing Program directed their ground-based telescopes towards the LP 791-18 system, facilitating the collection of observations pertaining to 67 transits involving planets c and d.
In the acquired data, the UdeM team successfully identified transit timing variations, which refer to subtle disparities in the precise timing of the planets' transits induced by the gravitational pull they exert on one another.
"This methodology enabled us to approximate the masses of planets c and d solely based on the transit data," elaborated Piaulet. "By amalgamating this information with the planets' sizes, which can be readily derived from the same data, we can estimate their density, offering insights into their composition and characteristics."
By juxtaposing these values with models of planetary interiors, astronomers successfully ascertained that the recently discovered planet possesses a mass akin to that of Earth. Consequently, its density aligns with that of a rocky composition, resembling Earth. On the other hand, planet c, boasting a mass approximately 7 times that of Earth, is presumed to have retained a notable quantity of gas or lighter substances, akin to the composition of Neptune.
'Crucial to analyse the atmosphere'
Planet c, being the largest in the system, has already secured observing time on the Webb Telescope under the Canadian NEAT program, which focuses on exoplanet research. Pierre-Alexis Roy, another Ph.D. student affiliated with Benneke's team at UdeM, will take the lead in analyzing these observations. "Accurately determining the mass of Planet c will be of utmost importance in examining the substantial atmosphere we anticipate discovering on this mini-Neptune," he clarifies.
In the future, the star's diminutive size may enable the potential detection of a less extensive atmosphere surrounding the newly discovered planet d. Scientists anticipate the possibility of an atmosphere resembling those found on Earth, Venus, or Saturn's moon Titan existing on Planet d. This system presents an unprecedented prospect for expanding our knowledge about small rocky planets, akin to the TRAPPIST-1 system that harbors seven Earth-sized planets, which is already under close scrutiny by the Webb Telescope. Consequently, it emerges as a prime target for Webb in the upcoming years, offering a comparable opportunity to delve into the intricacies of planetary systems, much like the TRAPPIST-1 system.
"This system offers astronomers a valuable laboratory for investigating and validating diverse hypotheses pertaining to the formation and progression of terrestrial planets," expressed Benneke. "The discovery of planet d, an Earth-sized world with a high likelihood of being cloaked in volcanoes within a multiplanetary system, presents unparalleled opportunities to make significant advancements not only in astronomy but also in numerous scientific domains, including geology, planetary sciences, atmospheric sciences, and potentially astrobiology."
About this study
“A temperate Earth-sized planet with tidally-heated interior transiting an M6 star” by Merrin Peterson et al., was published on May 17, 2023, in Nature.
In addition to Merrin Peterson, Björn Benneke, Caroline Piaulet, and Pierre-Alexis Roy, members of the Trottier Institute for Research on Exoplanets at Université de Montréal, the team also includes Jonathan Gagné, an UdeM adjunct professor and scientific ddvisor at the Space for Life Montreal Planetarium and member of iREx; Mohamad Ali-Dib, a former Trottier postdoctoral fellow at iREx, now at NYU Abu Dhabi; Ryan Cloutier, a former iREx graduate student and now assistant professor at McMaster University; Lauren Weiss, a former Trottier postdoctoral fellow at iREx who is now assistant professor at the University of Notre Dame; as well as 66 other co-authors from the United States, France, Spain, Belgium, Japan and Morocco.
Source
NASA Goddard Space Flight Center and the Trottier Institute for Research on Exoplanets – Université de Montréal
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