THERE IS NO EARTH B
The search for habitable planets expands
A University of Michigan astronomer and his team are suggesting a new way to expand the search for habitable planets that takes into account a zone not previously considered: the space between the star and what's called soot-line in planet-forming disks.
Worlds that form in this region—a disk of dust rotating around a central star from which planets may be built—could have surfaces rich in volatile carbon compounds quite different from Earth’s. These planets would also be rich in organic carbon, but water poor, according to Ted Bergin, who led the study that included geochemists, planetary scientists, astrochemists and exoplanet experts.
When we search for Earth-like planets, we are particularly interested not only in bodies that look like ours, but also in those that are formed by processes similar to ours. Current models of rocky exoplanets are built using Earth-like atmospheric conditions and bulk composition, including the molecules essential for life that form from carbon-based building blocks and water. These models also focus on zones within planet-forming disks called ice lines, regions distant enough from the disk's center star which mark where water or other key molecules transition from gas to solid phases.
Terrestrial worlds, like our planet, formed from solids. It has long been thought that Earth, which contains only approximately 0.1% water by mass, must have formed inside the water-ice line.
But that type of model may be too limited, Bergin said. To expand the search for habitable planets, Bergin and his research team suggest a new model that considers the soot line, a boundary closer to the solar system's star. Between this boundary and the star, organic compounds in solids sublimate out of the solid into gas. Considering this region would also encompass rocky planets that may have more carbon than Earth has, raising questions about what that means for habitability in these kinds of planets.
The findings by the interdisciplinary research team are published in Astrophysical Journal Letters.
"It adds a new dimension in our search for habitability. It may be a negative dimension or it may be a positive dimension," Bergin said. "It's exciting because it leads to all kinds of endless possibilities."
Just as Earth is poor in water, it is carbon poor as well, Bergin said. When forming, it likely received only 1 carbon atom per 100 available in planet-forming materials. Astronomers think the soot line explains why Earth has so little carbon. If Earth's building blocks formed inside the soot line, the temperature and solar radiation blasted the materials that would form the young planet, turning carbon-rich compounds into gas and limiting carbon in the solids that are supplied to the forming Earth.
The team's model theorizes about the formation of other planets born in between the soot line and water-ice lines.
Such a world does not appear to exist in our solar system, but our solar system is not representative of most known planetary systems around other stars, Bergin said. These other planetary systems look completely different. Their planets are closer to the sun and are much larger, ranging in size from what are called super-Earths to mini-Neptunes, he said.
"These are either big rocks or small gas giants—that's the most common type of planetary system. So maybe, within all those other solar systems out in the Milky Way galaxy, there exists a population of bodies that we haven't recognized before that have much more carbon in their interiors. What are the consequences of that?" Bergin said. "What this means for habitability needs to be explored."
In their study, the team models what happens when a silicate-rich world with 0.1% and 1% carbon by mass and a variable water content forms in the soot line region. They found that such a planet would develop a methane-rich atmosphere through a process called outgassing. In this circumstance, organic compounds in a silicate-rich planet produce a methane-rich atmosphere.
The presence of methane provides a fertile environment for the generation of hazes through interactions with stellar photons. This is analogous to the generation of hazes from methane in Titan in our own solar system.
"Planets that are born within this region, which exists in every planet-forming disk system, will release more volatile carbon from their mantles," Bergin said. "This could readily lead to the natural production of hazes. Such hazes have been observed in the atmospheres of exoplanets and have the potential to change the calculus for what we consider habitable worlds."
Haze around a planet might be a signpost that the planet has volatile carbon in its mantle. And more carbon, the backbone of life, in the mantle of a planet means that the planet has a chance to be considered habitable—or at least deserves a second glance, Bergin said.
"If this is true, then there could be a common class of haze planets with abundant volatile carbon, and what that means for habitability needs to be explored," he said. "But then there's the other aspect: What if you have an Earth-sized world, where you have more carbon than Earth has? What does that mean for habitability, for life? We don't know, and that's exciting."
Study: Exoplanet volatile carbon content as a natural pathway for haze formation
JOURNAL
The Astrophysical Journal Letters
Astronomers from the University of Liège and CSIC discover a key planetary system to understand the formation mechanism of the mysterious 'super-Earths'
The system, named TOI-2096, consists of two planets orbiting a cool star in a synchronized dance at approximately 150 light-years from Earth.
Peer-Reviewed PublicationIMAGE: ARTIST'S VIEW OF THE TOI-2096 SYSTEM view more
CREDIT: ©LIONEL J. GARCIA / ULIÈGE
A study led by researchers of the University of Liège and the CSIC - using observations from NASA's TESS telescope – presents the detection of a system of two planets slightly larger than Earth orbiting a cold star in a synchronized dance. Named TOI-2096, the system is located 150 light-years from Earth.
The discovery is the result of a close collaboration between European and American universities and was made possible by the US space mission TESS (Transiting Exoplanet Survey Satellite), which aims to find planets orbiting nearby bright stars. "TESS is conducting an all-sky survey using the transit method, that is, monitoring the stellar brightness of thousands of stars in the search for a slight dimming, which could be caused by a planet passing between the star and the observer. However, despite its power to detect new worlds, the TESS mission needs support from ground-based telescopes to confirm the planetary nature of the detected signals," explains Francisco J. Pozuelos, astrophysicist, first author of the paper, former member of the ExoTIC laboratory at the Univeristy of Liège, and who has now joined the Spanish National Research Council (IAA-CSIC).
The planets TOI-2096 b and TOI-2096 c were observed with an international network of ground-based telescopes, allowing their confirmation and characterization. The majority of the transits were obtained with telescopes of the TRAPPIST and SPECULOOS projects led by the University of Liège. “Making an exhaustive analysis of the data, we found that the two planets were in resonant orbits: for each orbit of the outer planet, the inner planet orbits the star twice," says Mathilde Timmermans, a doctoral student at the ExoTIC lab at ULiège and second author of the scientific paper. Their periods are therefore very close to being a multiple of each other with about 3.12 days for planet b and about 6.38 days for planet c. This is a very particular configuration, and it causes a strong gravitational interaction between the planets. This interaction delays or accelerates the passage of the planets in front of their star and could lead to the measurement of the planetary masses using larger telescopes in the near future.”
The researchers behind the discovery estimate that the radius of planet b - the closest to its star - is 1.2 times that of Earth, hence the name 'super-Earth'. Its properties could be similar to Earth’s: a planet with a mostly rocky composition, possibly surrounded by a thin atmosphere. Similarly, the radius of planet c is 1.9 times the radius of the Earth and 55% that of Neptune, which could place the planet in the category of 'mini-Neptunes', planets composed of a rocky and icy core surrounded by extended hydrogen- or water-rich atmospheres, such as Uranus and Neptune in our Solar System. These sizes are very interesting because the number of planets with a radius between 1.5 and 2.5 Earth radii is smaller than what theoretical models predict, making these planets a rarity. These planets are of crucial importance given their sizes," notes Mathilde Timmermans, "the formation of super-Earths and mini-Neptunes remains a mystery today. There are several formation models trying to explain it, but none fits the observations perfectly. TOI-2096 is the only system found to date that has a super-Earth and a mini-Neptune precisely at the sizes where the models contradict each other. In other words, TOI-2096 may be the system we've been looking for to understand how these planetary systems have formed.”
“Furthermore, these planets are among the best in their category to study their possible atmospheres," explains Francisco J. Pozuelos. Thanks to the relative sizes of the planets with respect to the host star, as well as the brightness of the star, we find that this system is one of the best candidates for a detailed study of their atmosphere with the JWST space telescope. We hope to be able to do this quickly by coordinating with other universities and research centers. These studies will help confirm the presence of an atmosphere, extensive or not, around planets b and c and thus give us clues as to their formation mechanism.”
JOURNAL
Astronomy and Astrophysics
ARTICLE TITLE
A super-Earth and a mini-Neptune near the 2:1 MMR straddling the radius valley around the nearby mid-M dwarf TOI-2096
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