An international group of collaborators, including scientists from NASA’s Jet Propulsion Laboratory and the University of New Mexico, have discovered a new temperate exoplanet the size of a sub-Neptune with an orbital period of 24 days in orbit around a neighboring dwarf star M. The recent discovery offers exciting research opportunities thanks to the planet’s substantial atmosphere, its small star, and the speed at which the system is moving away from Earth.
The research, titled TOI-1231 b: A Temperate, Neptune-Sized Planet Transiting the Nearby M3 Dwarf NLTT 24399, will be published in an upcoming issue of The Astronomical Journal. The exoplanet, TOI-1231b, was detected using photometric data from the Transiting Exoplanet Survey Satellite (TESS) and followed by observations using the Planet Finder Spectrograph (PFS) on the Magellan Clay telescope of the ‘Las Campanas Observatory in Chile. The PFS is a sophisticated instrument that detects exoplanets through their gravitational influence on their host stars. As the planets revolve around their hosts, the measured stellar speeds vary periodically, revealing the planetary presence and information about their mass and orbit.
The observation strategy adopted by NASA’s TESS, which divides each hemisphere into 13 sectors that are studied for approximately 28 days, produces the most comprehensive search in the sky for planets in transit. This approach has already proven its ability to detect both large and small planets around stars ranging from low mass M dwarf stars. M-dwarf stars, also known as red dwarfs, are the most common type of star in the Milky Way and make up about 70% of all stars in the galaxy.
M dwarfs are smaller and have a fraction of the mass of the sun and have low light. Because an M dwarf is smaller, when a planet of a given size passes through the star, the amount of light blocked by the planet is greater, making the transit more easily detectable. Imagine an Earth-like planet passing in front of a star the size of the sun, it will block out a tiny bit of light; but if it passes in front of a much smaller star, the proportion of blocked light will be greater. In a sense, this creates a larger shadow on the star’s surface, making the planets around the M dwarfs more easily detectable and easier to study.
Although it allows the detection of exoplanets in the sky, TESS’s survey strategy also produces significant observation biases based on the orbital period. Exoplanets must pass through their host stars at least twice in the TESS observation interval to be detected with the correct period by the Science Processing Operations Center (SPOC) pipeline and the Quick Look pipeline (QLP), which are looking for the 2 minute and 30- minute TESS cadence data, respectively. Because 74% of the total sky coverage of TESS is observed for only 28 days, the majority of TESS exoplanets detected have periods of less than 14 days. TOI-1231b’s 24-day period therefore makes its discovery even more valuable.
NASA scientist JPL Jennifer Burt, the lead author of the article, along with her collaborators including Diana Dragomir, assistant professor in the Department of Physics and Astronomy at UNM, measured both the radius and the mass of the planet.
“By working with a group of excellent astronomers around the world, we were able to put together the data needed to characterize the host star and measure both the radius and the mass of the planet,” Burt said. “These values in turn allowed us to calculate the bulk density of the planet and to make assumptions about the composition of the planet. TOI-1231 b is quite similar in size and density to Neptune, so we believe that it has an equally large gaseous atmosphere. “
“Another advantage of exoplanets orbiting M dwarf hosts is that we can measure their masses more easily because the ratio of the mass of the planet to the stellar mass is also greater. When the star is smaller and less massive, detection methods work better because the planet suddenly plays a bigger role as it stands out more easily from the star, ”Dragomir explained. “Like the shadow cast on the star. The smaller the star, the less massive the star, the more the effect of the planet can be detected.
“Even though TOI 1231b is eight times closer to its star than Earth is to the Sun, its temperature is similar to that of Earth, thanks to its colder and less bright host star,” explains Dragomir. “However, the planet itself is actually larger than Earth and a bit smaller than Neptune – we could call it a sub-Neptune.”
Burt and Dragomir, who actually started this research when they were fellows at MIT’s Kavli Institute, worked with scientists specializing in observing and characterizing the atmosphere of small planets to determine which space missions current and future might be able to scrutinize YOU. outer layers of 1231b to let researchers know exactly what types of gas are swirling around the planet. With a temperature of around 330 Kelvin or 140 degrees Fahrenheit, TOI-1231b is one of the coolest small exoplanets accessible for atmospheric studies discovered to date.
Previous research suggests that planets this cold can have clouds high in their atmosphere, making it difficult to determine what types of gases surround them. But new observations of another small, cold planet called K2-18 b broke that trend and showed traces of water in its atmosphere, surprising many astronomers.
“TOI-1231b is one of the only other planets known to us in a similar size and temperature range, so future observations of this new planet will allow us to determine how common (or rare) it is for clouds of water form around these temperate worlds, ”said Burt.
In addition, with the high brightness of its near infrared (NIR) star, it is an attractive target for future missions with the Hubble Space Telescope (HST) and the James Webb Space Telescope (JWST). The first round of sightings, led by one of the paper’s co-authors, is expected to take place later this month using the Hubble Space Telescope.
“The low density of TOI 1231b indicates that it is surrounded by a substantial atmosphere rather than being a rocky planet. But the composition and extent of this atmosphere is unknown!” said Dragomir. “TOI1231b could have a large atmosphere of hydrogen or hydrogen-helium, or a denser water vapor atmosphere. Each of these would indicate a different origin, allowing astronomers to understand if and how planets form differently around M dwarfs compared to planets around our Sun, for example. Our next HST observations will begin to answer these questions, and JWST promises an even closer examination of the planet’s atmosphere.
Another way to study the planet’s atmosphere is to determine if gas is being blown away, looking for evidence of atoms like hydrogen and helium surrounding the planet as it passes the star. host. Usually, hydrogen atoms are almost impossible to detect because their presence is obscured by interstellar gas. But this planet-star system offers a unique opportunity to apply this method due to the speed at which it is moving away from Earth.
“One of the most intriguing findings from the past two decades of exoplanet science is that, so far, none of the new planetary systems we’ve discovered resemble our own solar system,” Burt said. “They are full of planets between the size of Earth and Neptune in orbits much shorter than Mercury, so we don’t have any local examples to compare them with. This new planet that we have discovered is still there. strange – but it’s one that gets closer and closer to being a bit like our neighboring planets. Compared to most of the transit planets detected so far, which often have scorching temperatures of several hundred or thousand degrees , TOI-1231 b is positively icy.
In conclusion, Dragomir explains that “this planet joins the ranks of only two or three other neighboring small exoplanets which will be scanned at every opportunity and using a wide range of telescopes, for years to come, so keep an eye out for the new TOI1231b evolutions! “