Image: A gas giant exoplanet [right] with the density of a marshmallow has been detected in orbit around a cool red dwarf star [left]. The planet, named TOI-3757 b, is the fluffiest gas giant planet ever discovered around this type of star. Credit: NOIRLab/ NSF/ AURA/ J da Silva/ Spaceengine/ M Zamani(CC BY 4.0) https://creativecommons.org/licenses/by/4.0/legalcode.
To announce the discovery of a new type of planet is impressive. To announce another completely different type of planet just four months later, is bordering on the amazing. But that is just what Shubham Kanodia of the Carnegie Institution for Science, and collaborators across the US, have done. Aether takes a look at his discovery of a ‘Marshmallow planet’ due to its density, and another ‘forbidden planet’ that shouldn’t really exist at all…
PLANET ONE: ‘Marshmallow’ world orbiting a cool red dwarf star
In October of last year, it was announced the Hobby-Eberly Telescope (HET) in Texas, and the Kitt Peak National Observatory in Arizona, had helped to determine that a Jupiter-like planet was lowest-density gas giant ever detected around a red dwarf.
The gas giant exoplanet with the density of a marshmallow was detected orbiting a cool red dwarf star by a suite of instruments, including the Habitable zone Planet Finder (HPF) on the 10m HET at McDonald Observatory, the NASA-funded NEID radial-velocity instrument on the WIYN 3.5m telescope at Kitt Peak National Observatory, a programme of NSF’s NOIRLab. The planet, named TOI-3757 b, is located approximately 580 light-years from Earth in the constellation of Auriga the Charioteer, and is the fluffiest gas giant planet ever discovered around this type of star.
Red dwarf stars are the smallest and faintest members of so-called main sequence stars – stars that convert hydrogen into helium in their cores at a steady rate. Though ‘cool’ compared to stars like our sun, red dwarf stars can be extremely active and erupt with powerful flares capable of stripping a planet of its atmosphere, making this star system a seemingly inhospitable location to form such a gossamer planet.
Shubham Kanodia, a researcher at Carnegie Institution for Science’s Earth and Planets Laboratory and first author on a paper, said: “Giant planets around red dwarf stars have traditionally been thought to be hard to form. So far this has only been looked at with small samples from Doppler surveys, which typically have found giant planets further away from these red dwarf stars. Until now we have not had a large enough sample of planets to find close-in gas planets in a robust manner.”
Image: The Hobby-Eberly Telescope (HET) is a 10m telescope, and has been primarily responsible for the discovery of both the ‘marshmallow’ and ‘forbidden’ planets. Credit: Ethan Tweedie/ The University of Texas at Austin.
There are still unexplained mysteries surrounding TOI-3757 b, the big one being how a gas-giant planet can form around a red dwarf star, and especially such a low-density planet. Kanodia’s team, however, thinks they might have a solution.
They propose that the extra-low density of TOI-3757 b could be the result of two factors. The first relates to the rocky core of the planet; gas giants are thought to begin as massive rocky cores about ten times the mass of Earth, at which point they rapidly pull in large amounts of neighbouring gas to form the gas giants we see today. TOI-3757 b’s star has a lower abundance of heavy elements compared to other M-dwarfs with gas giants, and this may have resulted in the rocky core forming more slowly, delaying the onset of gas accretion and therefore affecting the planet’s overall density.
The second factor may be the planet’s orbit, which is tentatively thought to be slightly elliptical. There are times it gets closer to its star than at other times, resulting in substantial excess heating that can cause the planet’s atmosphere to bloat.
Crossing in front of its star
The planet was initially spotted by NASA’s Transiting Exoplanet Survey Satellite (TESS). Kanodia’s team then made follow-up observations using ground-based instruments, including NEID and NESSI (NN-EXPLORE Exoplanet Stellar Speckle Imager), both housed at the WIYN 3.5-meter Telescope; the Habitable-zone Planet Finder on the Hobby-Eberly Telescope; and the Red Buttes Observatory (RBO) in Wyoming.
TESS surveyed the crossing of this planet TOI-3757 b in front of its star, which allowed astronomers to calculate the planet’s diameter to be about 150,000 kilometres (100,000 miles) or about just slightly larger than that of Jupiter. The planet finishes one complete orbit around its host star in just 3.5 days, 25 times less than the closest planet in our solar system – Mercury – which takes about 88 days to orbit our sun.
The astronomers then used HPF and NEID to measure the star’s apparent motion along the line of sight, also known as its radial velocity. These measurements provided the planet’s mass, which was calculated to be about one quarter that of Jupiter, or about 85 times the mass of the Earth. Knowing the size and the mass allowed Kanodia’s team to calculate TOI-3757 b’s average density as being 0.27 grams per cubic centimetre (about 17 grams per cubic feet), which would make it less than half the density of Saturn (the lowest-density planet in the solar system), about one quarter the density of water (meaning it would float if placed in a giant bathtub filled with water), or in fact, similar in density to a marshmallow.
Jessica Libby-Roberts, a postdoctoral researcher at Pennsylvania State University and the second author on this paper, said: “Potential future observations of the atmosphere of this planet using NASA’s new James Webb Space Telescope could help shed light on its puffy nature.”
Kanodia added: “Finding more such systems with giant planets – which were once theorised to be extremely rare around red dwarfs – is part of our goal to understand how planets form.”
The discovery highlights the importance of HPF and NEID in its ability to confirm some of the candidate exoplanets currently being discovered by NASA’s TESS mission, providing important targets for the new James Webb Space Telescope (JWST) to follow-up on and begin characterising their atmospheres. This will in turn inform astronomers what the planets are made of and how they formed and, for potentially habitable rocky worlds, whether they might be able to support Life.
The paper is published in The Astronomical Journal.
Image: The WIYN 3.5m telescope. Credit: KPNO/ NOIRLab/ NSF/ AURA/ T Matsopoulos (CC BY 4.0) https://creativecommons.org/licenses/by/4.0, via Wikimedia Commons.
PLANET TWO: Gas giant formation theories challenged by ‘forbidden planet’
A ‘forbidden planet’ orbiting a small star is challenging gas giant formation theories according to US researchers. Based on our current understanding of planet formation, TOI-5205b should not exist; it is a ‘forbidden planet’, according to a team of astronomers led by the Carnegie Institution for Science’s Shubham Kanodia.
The team has discovered an unusual planetary system in which a large gas giant planet orbits a small red dwarf star called TOI-5205. Their findings challenge long-held ideas about planet formation.
Smaller and cooler than our sun, M dwarfs are the most common stars in our Milky Way galaxy. Due to their small size, these stars tend to be about half as hot as the sun and much redder.
They have very low luminosities, but extremely long lifespans. Although red dwarfs host more planets, on average, than other, more massive types of stars, their formation histories make them unlikely candidates to host gas giants. The newly discovered planet – TOI 5205 b – was first identified as a potential candidate by NASA’s Transiting Exoplanet Survey Satellite (TESS). Kanodia’s team then confirmed its planetary nature and characterised it using a variety of ground-based instruments and facilities.
Kanodia, who specialises in studying these stars, which comprise nearly three-quarters of our galaxy, yet can’t be seen with the naked eye, said: “The host star, TOI-5205, is just about four times the size of Jupiter, yet it has somehow managed to form a Jupiter-sized planet, which is quite surprising!”
A small number of gas giants have been discovered orbiting older M dwarf stars. But until now no gas giant has been found in a planetary system around a low-mass M dwarf like TOI-5205. To grasp the size comparison here, a Jupiter-like planet orbiting a sun-like star could be compared to a pea going around a grapefruit; for TOI-5205 b, because the host star is so much smaller, it is more like a pea going around a lemon.
Image: “A Jupiter-like planet orbiting a sun-like star could be compared to a pea going around a grapefruit; for TOI-5205b, because the host star is so much smaller, it is more like a pea going around a lemon,” said lead author Shubham Kanodia. Credit: Artwork by Katherine Cain, courtesy of the Carnegie Institution for Science.
In fact, when the Jupiter-mass TOI 5205 b crosses in front of its host, it blocks about 7% of its light – one of the largest known exoplanet transits. Planets are born in the rotating disk of gas and dust that surrounds young stars.
The most commonly used theory of gas planet formation requires about ten Earth masses of this rocky material to accumulate and form a massive rocky core, after which it rapidly sweeps up large amounts of gas from the neighbouring regions of the disk to form the giant planet we see today.
The time frame in which this happens is crucial.
Kanodia explained: “TOI-5205 b’s existence stretches what we know about the disks in which these planets are born.
“In the beginning, if there isn’t enough rocky material in the disk to form the initial core, then one cannot form a gas giant planet. And at the end, if the disk evaporates away before the massive core is formed, then one cannot form a gas giant planet. And yet TOI-5205 b formed despite these guardrails.
“Based on our nominal current understanding of planet formation, TOI-5205 b should not exist; it is a ‘forbidden’ planet.”
The team demonstrated that the planet’s very large transit depth makes it extremely conducive for future observations with the recently launched JWST, which will shed some light on its atmosphere and offer some additional clues about the mystery of its formation.
The findings are published in The Astronomical Journal.
Image: Artist’s conception of a large gas giant planet orbiting a small red dwarf star called TOI-5205. Until now no gas giant has been found in a planetary system around a low-mass M dwarf like TOI-5205. Credit: Image by Katherine Cain, courtesy of the Carnegie Institution for Science.
Aether talks to Shubham Kanodia…
Aether: Can you tell me a little bit about your previous research and how it led to this work?
SK: I have been lucky enough to be part of a wonderful team involved in building two Doppler spectrographs to find planets around M-dwarfs (Habitable-zone Planet Finder) and solar type stars (NEID) during my PhD at Pennsylvania State University. Now, along with my collaborators, we are using these spectrographs to find more planets, characterise them, and understand how they form.
Aether: There must be billions of M dwarf stars – how do you decide which to study? Do you have a favourite?
SK: There are indeed a LOT of M-dwarfs in the galaxy (hundreds of billions)! In fact they form about 75% of the stars in the galaxy. They are small, red, and everywhere! Right now we utilise data from NASA’s TESS mission which observes (almost) the entire sky every two years looking for transiting planets. By staring at a patch of the sky continuously for about 27 days at a time, it finds periodic dips in the light from the star that might be indicative of a transiting object. We look at these transiting planet candidates around M-dwarfs, while focusing on the giant planets (like Jupiter).
Aether: How long did it take from Nasa’s TESS identifying TOI-5205 to you discovering this ‘unusual planetary system’?
SK: I think we started following-up this object a couple months after it was identified as a candidate through TESS data, and from there-on took about six months before we had all the data and submitted the article for peer-review.
Aether: A small number of gas giants have been discovered orbiting older M dwarf stars. But until now no gas giant has been found in a planetary system around a low-mass M dwarf like TOI-5205. What is so different?
SK: That is correct. The host-star for TOI-5205 b is much smaller than the others known to host similar giant planets at just about 40% the mass of our sun. We think these systems must be quite rare, but must find more of these to truly quantify how rare.
Aether: Are you confident of securing time using the JWST to study this star and its system?
SK: JWST is the fancy new toy on the block for astronomers, and everyone wants to use it for their favourite science because of the revolutionary promise it offers. Therefore, the over-subscription for JWST time is quite high. That being said, we certainly hope so!
[ADDENDUM 10/05/2023: We did indeed receive JWST time to observe both TOI-3757 and TOI-5205 in Cycle 2.]