Astronomers have discovered an unusual planetary system consisting of a planet the size and weight of slightly more than Jupiter orbiting a tiny star, TOI-5205b, which is little more than the solar system’s gas giant.
This “forbidden” configuration of a huge planet orbiting a relatively small star it could challenge theories about how gas giant planets formjust think that the planet is 40 percent of the mass of the Sun, with a ratio of the mass of the planet to the mass of the star which is 0.3 percentthe tallest of all known planets orbiting red dwarfs.
Red dwarfs are the most common type of star out there, are smaller and dimmer than our Sun and live much longer. They’re also an ideal place for planet formation, but not for all planet types: Current models suggest they’re an unlikely place for gas giant formation, so imagine the surprise when astronomers found planet TOI-5205b .
The extrasolar planet, or “exoplanet”, as already mentioned orbits the red dwarf star TOI-5205b which is much cooler and smaller than the sun, furthermore the small size and relatively low temperatures of these M dwarf stars – the most common of stellar body in the Milky Way– make them redder than the sun.
Although on average this class of stars harbors more planets than other types of stars, their formation was previously believed to make it unlikely that they would be orbited by gas giants. The discovery of exoplanet TOI-5205b by astronomers using NASA’s Transiting Exoplanet Survey Satellite (TESS) telescope calls this notion into question. The planet was confirmed and characterized by the team using various telescopes and ground-based instruments.
“The host star, TOI-5205, is about four times the size of Jupiter, yet it somehow managed to form a Jupiter-sized planet, which is quite amazing!”
he said in a statement Shubham Kanodia, team leader and Carnegie Science astronomer.
The extraordinary nature of TOI-5205b and its planetary system
Although gas giants have been discovered previously around M dwarf stars, none of them have been discovered orbiting such a low-mass example of this class of star-like TOI-5205b.
Planets are created in rotating disks of gas and dust called “protoplanetary disks” that surround young stars. This material is the remnants of the same matter that collapsed to form its central star. As the dense spots collapse under their own gravity, the cores of the planets are born and then collect more material.
Planets form in the circumstellar disk, a disk of matter that surrounds newborn stars. A planet of this size would have to accumulate a rocky core of about 10 Earth masses and subsequently obtain enough gas to reach something close to 320 times our planet’s mass. And the models didn’t expect this scenario to play out around a red dwarf.
Current models of planet formation suggest that it takes material equivalent to 10 times the mass of Earth to give birth to a gas giant. This first forms a rocky core and this core continues to accumulate gas to form the disk to grow a giant planet. However, this process must happen quickly.
“In the beginning, if there isn’t enough rocky material in the disk to form the initial core, then a gas giant planet cannot be formed. And finally, if the disk evaporates before the massive core is formed, then one cannot form a gas giant planet. Yet TOI-5205b formed despite these guardrails.
Based on our current nominal understanding of planet formation, TOI-5205b shouldn’t exist; it’s a ‘forbidden’ planet.”
Kanodia explained in the statement.
To imagine how unbalanced this system is compared to the planetary systems astronomers have come to expect, imagine our star the sun squeezed down to the size of a grapefruit. Such a reduction in size would mean that the largest gas giant in our solar system, Jupiter, would be about the size of a pea.
The TOI-5205 system is more like a pea orbiting a lemon.
The size disparity in size is so great that when TESS used the light drop-off caused by a planet as it passes in front of its star, known as the transit method, that light drop-off was 7% of the star’s total light output. .
The planet was first spotted by the Transiting Exoplanet Survey Satellite (TESS). It was then followed up by the Kanodia team and they were able to confirm that it was indeed a planet and work out some of its properties. The planet was discovered by the transit method, by observing the light from its star. TESS measured a dip in light as the planet passed in front of it. And it was quite a dive. Seven percent of the light was blocked, one of the largest known exoplanet transits
This makes the dimming of TOI-5205 by this Jupiter-sized exoplanet the largest known dip in light caused by an exoplanet transit.
This extreme light dropout, or technically “large transit depth” bodes well for future observations, could make the system ideal for follow-up surveys with the James Webb Space Telescope (JWST).
Observations with the JWST could help determine the composition of TOI-5205’s atmosphere and could shed light on the processes that gave birth to this “forbidden” planet.
Details of this discovery are given in the The Astronomical Journalwhich as usual explains the fantastic discovery in a more in-depth and technical way.
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