Sub-Planet Neptune orbits in the “habitable zone” of a red dwarf star

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Artist’s impression of planets orbiting a red dwarf. Credit: Mark Garlick

An international research team led by the University of Bern has discovered a sub-exoplanet of Neptune orbiting a red dwarf star. The discovery was also made thanks to observations made by the SAINT-EX observatory in Mexico. SAINT-EX is operated by a consortium that includes the Center for Space and Habitat (CSH) at the University of Bern and the National Center for Competence in Research by NCCR PlanetS.

Red dwarfs are small stars, and therefore cooler than our Sun. Around stars like these, liquid water is possible on planets closer to the star than in our own solar system. The distance between an exoplanet and its star is a determining factor in its discovery: the closer a planet is to its host star, the more likely it is to be detected.

In a study recently published in the Journal of Astronomy and Astrophysics, researchers led by Dr. Nicole Chanch of the Center for Space and Habitability at the University of Bern report the discovery of an exoplanet TOI-2257 b orbiting a nearby red dwarf. Nicole Schanci is also a member of the National Center for Competence in Planetary Research, which is run by the University of Bern together with the University of Geneva.

A special telescope is part of the solution

Exoplanets that are too far from our solar system cannot be observed directly with a telescope – they are too small and reflect very little light. However, one of the ways to discover these planets is the transit method. This involves using telescopes to look for dips in a star’s brightness that occur when planets pass in front of the star. Repeated observations of dips in the star’s brightness give accurate measurements of the planet’s orbital cycle around the star, and the depth of transit allows researchers to determine the planet’s diameter. When combined with estimates of a planet’s mass from other methods, such as using radial velocity measurements, a planet’s density can be calculated.

The planet TOI-2257 b was initially identified by data from NASA’s TESS Transiting Exoplanet Space Telescope. The young star was observed for four months, but the gaps between the observations meant that it was not clear whether the decrease in brightness could be explained by transits of a planet with an orbit of 176, 88, 59, 44 or 35 days.

The SAINT-EX Observatory is a fully automated facility that hosts a one-meter telescope and is based in Mexico. Credit: Institute of Astronomy, UNAM / E. Cadena

Observing the star with the Las Cumbres Observatory Global Telescope later ruled out the possibility that the planet with an orbital period of 59 days had caused the decrease in brightness. “Next, we wanted to see if a 35-day orbital period was possible,” Nicole Shanshi explains.

The SAINT-EX telescope based in Mexico, in collaboration with CSH and NCCR PlanetS, has been designed for the purpose of studying red dwarfs and their planets in more detail. SAINT-EX is an acronym that stands for Search and Characterization of Transiting Exoplanets. The project is named in honor of Antoine de Saint-Exupery (Saint X), the famous writer, poet and aviator. SAINT-EX observed a partial transit of TOI-2257 b and was able to confirm the exoplanet’s exact orbital period around its star, 35 days. “After another 35 days, SAINT-EX was able to monitor the entire transit, which gave us more information about the system’s characteristics,” says co-author Robert Wells of CSH, who was involved in processing the data.

A temperate planet with an irregular orbit

With an orbital period of 35 days, TOI-2257 b orbits the host star at a distance at which liquid water is possible on the planet, and therefore favorable conditions for the emergence of life can exist. Planets in the so-called “habitable zone” near a small red dwarf are easy to study because their orbit periods are shorter and therefore they can be observed more often. The radius of TOI-2257 b (2.2 times larger than Earth) indicates that the planet is rather gaseous, with high atmospheric pressure not conducive to life.

TESS target pixel files for sectors 14, 20, 21 and 26 observed by TOI-2257, which were generated by tpfplotter (Aller et al. 2020). Slots used for photometric extraction by the SPOC pipeline are shown as shaded regions in red. The Gaia DR2 catalog (Gaia Collaboration 2018) is exaggeratedly plotted, with all sources up to 6 magnitudes unlike TOI-2257 shown as red circles. We note that the symbol size is proportional to the size variance. While the star is relatively isolated, there is a small amount of pollution from external sources, ranging from 2-5% of the total flux. Credit: DOI: 10.1051 / 0004-6361 / 202142280

“We found that TOI-2257 b does not have a circular, concentric orbital,” Nicole Shansch explains. In fact, it’s the most exotic planet orbiting a cold star ever discovered. “In terms of habitability, that’s bad news,” Nicole Shanchy continues. “While the average planet temperature is comfortable, it ranges from -80 degrees Celsius to about 100 degrees Celsius depending on where the planet is in its orbit, far from or close to the star.” A possible explanation for this surprising orbit is that a giant planet in the system lurks and disturbs the orbit of TOI 2257 b. Additional observations measuring the star’s radial velocity will help confirm the central eccentricity and search for possible additional planets that cannot be observed during transits.

Filter for monitoring with JWST

The James Webb Space Telescope (JWST), which launched successfully on December 25, will revolutionize the search for the atmospheres of exoplanets. In order to prioritize good candidates for observations using JWST, a transmission spectrophotometer (TSM) that classifies various system properties has been developed. The TOI-2257 b is well positioned with respect to TSM and is one of Neptune’s most attractive sub-targets for further observations. “In particular, the planet can be studied for signs of features such as water vapor in the atmosphere,” Nicole Shansh concludes.

Reference: “TOI-2257 b: Eccentric long-range sub-Neptune passes through a nearby M dwarf” by N. Schanche, F. J. Pozuelos, MN Günther, RD Wells, A. J. Burgasser, P. Chinchilla, L. Delrez, and E. Ducrot, LJ Garcia, Y. Gómez, Maqueo Chew, E. Jofré, B. V. Rackham, D. Sebastian, K. G. Stassun, D. Stern, M. Timmermans, K. Barkaoui, A. Belinski, Z. Benkhaldoun, W. Benz, A. Perilla, F. Boshi, A.; Bordanov, D.; Charbonneau, J.L. Christiansen, CA Collins, B.-O. Demore, M.; Devora Bagaris, c. De Witt, Dr.; Dragomir, c. Dansfield, E. Forlan, M. Gaschoy, M. Gillon, C. Jenelka, M.A. K. Heng, C. E. Henze, K. Hesse, S. B. Howell, E. Jehin, J. Jenkins, E. N. Jensen, M. Kunimoto, D. W. Latham, K. Lester, K. McLeod, I.Mireles, C. A. Murray, P. Niraula, P. P. Pedersen, D. Queloz, E. V. Quintana, G. Ricker, A. Rudat, L. Sabin, B. Safonov, U. Schroffenegger, N. Scott, S. Seager, I. Strakhov, AHMJ Triaud, R. Vanderspek, M . Vezie and J. Winn, 7 January 2022, Astronomy & Astrophysics.DOI: 10.1051/0004-6361/202142280

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