March 1, 2024

A super-Earth (and possible Earth-sized) exoplanet found in the habitable zone

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Artist’s representation of the surface of a super-Earth orbiting a red dwarf. Credit: ESO/M. Kornmesser

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Artist’s representation of the surface of a super-Earth orbiting a red dwarf. Credit: ESO/M. Kornmesser

Astronomers have discovered a new Super-Earth orbiting an M dwarf (red dwarf) star about 137 light-years away. The planet is called TOI-715b, it is about 1.55 the radius of Earth and is within the star’s habitable zone. There is also another planetary candidate in the system. It is the size of Earth and, if confirmed, would be the smallest habitable zone the planet TESS has discovered so far.

TOI-715 is a medium red dwarf. It is about a quarter of the mass and about a quarter of the radius of our sun. TOI-715b is close to the star and its tight orbit takes just 19 days to complete one trip around the dwarf star. Because red dwarfs are much dimmer than the Sun, this places Super-Earth in the star’s conservative habitable zone.

New research published in Monthly Notices of the Royal Astronomical Society presents the discovery. It is titled “A habitable zone planet with Earth radius of 1.55 hosted by TOI-715, an M4 star near the ecliptic South Pole.” The lead author is Georgina Dransfield, from the School of Physics and Astronomy at the University of Birmingham.

The habitable zone is a very crude way of identifying planets that might have liquid water. Its limits are unclear and even contradictory, since the stellar spectral type, planetary albedo, mass and even how cloudy its atmosphere is can determine whether a planet has liquid water.

The idea of ​​a conservative habitable zone (CHZ) is more useful. It comes from a 2014 paper by Kopparapu et al. It is a region around a star where a rocky planet receives between 0.42 and 0.842 as much solar insolation as Earth. Any rocky planet that receives this much energy is in the CHZ, regardless of distance.

Discovering a Super-Earth in a star’s CHZ is always exciting. It feeds our sense of wonder about other planets and the possibility that some could harbor other forms of life. For this reason, they are more intriguing than planets like Hot Jupiters, for example, which have no possibility of hosting liquid water or life. Not even the most hardy extremophiles can survive Hot Jupiter’s wicked environment.

But this discovery is also exciting for a few other reasons.

Now that we have discovered thousands of exoplanets, astronomers are observing trends in the population. One of the things they noticed was a gap in the small planet’s population between 1.5 and 2 Earth radii. It is known as the small planet radius gap or sub-Neptune radius gap (also called the Fulton gap and photoevaporation valley). At 1.55 Earth radii, TOI-715b lies within the gap.

It is extremely unlikely that any planets will form in this radius gap. Planets must start out bigger and lose mass to end up in the gap. So the Fulton Gap tells us something about how some planets lose mass. Astronomers think the planets in the gap start out larger, but their stars lose some of their mass through photoevaporation, shrinking them. This is why it is sometimes called the trough of photoevaporation. There is a lot of uncertainty surrounding the valley and photoevaporation, and astronomers want to study the planets in the valley to see what they can learn.


This figure from the study shows the conservative habitable zone in blue. The y-axis shows the Transmission Spectroscopy Metric, a measure of how receptive a planet’s atmosphere is to JWST spectroscopic measurements. The x-axis shows stellar insolation. TOI-715b is shown in two cases: as a rocky world and as a watery world. If it is a water world, it is more accessible to effective JWST spectroscopy. TOI-715’s other planet candidate is shown in green. Other interesting exoplanets are also shown, including the TRAPPIST-1 planets in yellow. Credit: Monthly Notices of the Royal Astronomical Society (2023). DOI: 10.1093/mnras/stad1439

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This figure from the study shows the conservative habitable zone in blue. The y-axis shows the Transmission Spectroscopy Metric, a measure of how receptive a planet’s atmosphere is to JWST spectroscopic measurements. The x-axis shows stellar insolation. TOI-715b is shown in two cases: as a rocky world and as a watery world. If it is a water world, it is more accessible to effective JWST spectroscopy. TOI-715’s other planet candidate is shown in green. Other interesting exoplanets are also shown, including the TRAPPIST-1 planets in yellow. Credit: Monthly Notices of the Royal Astronomical Society (2023). DOI: 10.1093/mnras/stad1439

“The importance of the radius valley lies in its potential to teach us about planetary formation and post-formation evolution, and therefore the planets within this gap are crucial to deepening our understanding of the factors that sculpt it,” the authors explain. .

There is some uncertainty whether this radius gap exists around M dwarfs or not. It is possible that M dwarfs have a density gap rather than a radius gap. “A recent study by Luque & Pallé (2022), however, indicates that M dwarf planets may have a density gap rather than a radius gap that separates two populations of small planets (rocky and water worlds),” they write the authors.

Whether it’s a radius gap or a density gap, TOI-715b should have something to tell us about exoplanets, photoevaporation, and the nature of exoplanet distribution around red dwarfs. But discovering what he has to tell us requires more detailed observations. That’s the second reason why this Super-Earth is so intriguing.

Ever since we started finding exoplanets, scientists have eagerly awaited the day when the James Webb Space Telescope will be operational. “The era of JWST has finally arrived and, with it, the era of detailed exoplanet atmospheric characterization,” the authors write in their paper. JWST has the ability to observe the spectra of exoplanet atmospheres and determine their constituents. But although JWST is extremely powerful, some targets present better opportunities for transmission spectroscopy than others.

TOI-715b is a prime target because it is close to its star. As TOI-715 is a small red dwarf and the planet orbits it every 19 days, the exoplanet’s transits in front of its star are deeper and more frequent. This means JWST doesn’t need much time to observe the planet’s atmosphere, making it an efficient use of the space telescope’s time.

“In the context of atmospheric characterization by transmission spectroscopy, bright and nearby M dwarfs are ideal planetary hosts, as small temperate planets will transit frequently, allowing detections of atmospheric features with high signal-to-noise ratios with fewer hours of telescope time,” explain the authors. .

Could this Super-Earth be habitable? In the absence of JWST spectroscopy, we are left to speculate. It’s in the conservative habitable zone, but that doesn’t get us very far. Still, there are some signs of hope.

TOI-715 is a little older than our Sun, at about 6.6 billion years old. The star has a “low degree of magnetic activity”, according to the authors. This is probably why the star shows an absence of brightness in the TESS light curves compared to younger M dwarfs. Red dwarfs are known for exhibiting extremely powerful explosions that can sterilize planets. They can also destroy atmospheres, which could be responsible for the trough of photoevaporation of exoplanets.

Another planet may be orbiting TOI-715. It’s currently just one candidate called TIC 271971130.02, but if confirmed, it will be the smallest habitable zone the planet TESS has ever encountered. But follow-up observations are needed to confirm it.

The TOI-715 system is an attractive target for further study. TOI-715b is waiting its turn, but eventually JWST will examine its atmosphere. If these results support habitability, astronomers’ enthusiasm will only increase. At the same time, we may learn more about the radius or density gap, an obstacle to deeper understanding of exoplanets.

Add in the fact that the star may host another habitable zone planet, the smallest ever found by TESS, and the TOI-715 system becomes even more important.

More information:
Georgina Dransfield et al, A 1.55 R⊕ habitable zone planet hosted by TOI-715, an M4 star near the ecliptic South Pole, Monthly Notices of the Royal Astronomical Society (2023). DOI: 10.1093/mnras/stad1439

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