The planet hunter NASA's Transiting Exoplanet Survey Satellite (TESS) has discovered three unknown planets to date: one slightly larger than Earth and two about half the size of Neptune. The three stars, baptized as TOI-270b, c and d, orbit a nearby star (at 73 light years), as published Nature Astronomy
The host star of the system is TOI-270, a M-type dwarf (slightly bright and cold) that is approximately 40% smaller than the Sun in both size and mass and has a cooler third surface. The planetary system is in the southern constellation of Pictor.
"This system is exactly what the TESS: small, temperate planets that pass or pass in front of an inactive host star, which lacks excessive stellar activity, such as flares, "says lead researcher Maximilian Günther of the Kavli Institute for Astrophysics and Space Research at the Massachusetts Institute of Technology (MIT ) in Cambridge.
"This star is silent and very close to us, and therefore much brighter than the host stars of comparable systems. With extensive follow-up observations, we will soon be able to determine the composition of these worlds, establish if they have atmospheres and what gases they contain." , point.
A rocky world
The innermost planet, called TOI 270 b, is probably a rocky world 25% larger than Earth. The star orbits every 3.4 days at a distance approximately 14 times closer than Mercury of the Sun. Based on statistical studies of known exoplanets of similar size, the scientific team estimates that TOI 270 b has a mass about 1.9 times greater than Earth's
Because of its proximity to the star, planet b is a hot world like an oven. Its equilibrium temperature – that is, the temperature based solely on the energy it receives from the star, which ignores the additional heating effects of a possible atmosphere – is around 255 degrees Celsius.
The other two planets, TOI 270 c and d, are, respectively, 2.4 and 2.1 times larger than Earth and orbit the star every 5.7 and 11.4 days. Both can be similar to Neptune in our solar system, with compositions dominated by gases instead of rocks, and probably weigh about 7 and 5 times the mass of the Earth, respectively.
"An interesting aspect of this system is that its planets extend to both sides of a well-established gap in known planetary sizes," says Francisco Pozuelos, a postdoctoral researcher at the University of Liege in Belgium and a collaborating researcher at the University of Granada.
An excellent laboratory
"It is uncommon for planets to have sizes between 1.5 and two times larger than those of the Earth, for reasons probably related to the way planets are formed, but this is a very controversial issue. TOI 270 is a excellent laboratory to study the margins of this gap, and it will help us to better understand how they form and evolve planetary systems, "says Pozuelos.
Planets c and d could best be described as ‘mini-Neptunes’, a type of planet that is not seen in our solar system. The researchers hope that further exploration of the TOI 270 can help explain how two of these mini-Neptunes formed alongside a world almost the size of the Earth.
Planets b and c orbit at a resonance close to 5: 3, which means that the inner planet surrounds the star five times for every three orbits of the furthest. Planets c and d have a similar relationship, with d taking almost twice as long to orbit the star as c. This arrangement will allow astronomers to look for slight variations in the time of their transits, which could provide more information about the masses of the planets and the general dynamics of the system.
Günther's team is particularly interested in the outermost planet, TOI 270 d. The team estimates that the planet's equilibrium temperature is about 66 degrees Celsius. This makes it the most temperate world in the system and, as such, a rarity among known transit planets.
"The TOI 270 is perfectly located in the sky to study the atmospheres of its outer planets with NASA's future James WebbSpaceTelescope," said co-author Adina Feinstein, a PhD student at the University of Chicago. "It will be observed by the Webb for more than half a year, which could allow really interesting comparison studies between the atmospheres of TOI 270 c and d."
Presence of liquid water?
The team hopes that additional research can reveal additional planets beyond the three now known. If planet d has a rocky core covered by a thick atmosphere, its surface would be too hot for the presence of liquid water, considered a key requirement for a potentially habitable world.
But follow-up studies may discover additional rocky planets slightly longer distances from the star, where colder temperatures could allow liquid water to accumulate on their surfaces. "Due to the proximity and configuration of these planets, the system is perfect for our dynamics analysis, which will help to discover if there are indeed additional planets in the system," says the researcher at the University of Granada Juan Carlos Suárez.
“In addition, the three planets found so far are expected to be synchronized with the star, which means that they only rotate once in each orbit and keep one side facing the star, just as the Moon does in its orbit around the Earth ”, comment Pozuelos and Suárez.
TESS is a NASA astrophysical exploration mission run and operated by MIT in Cambridge, Massachusetts, and administered by NASA's Goddard Space Flight Center. Other partners are Northrop Grumman, based in Falls Church, Virginia; NASA's Ames Research Center in Silicon Valley, California; the Harvard-Smithsonian Astrophysics Center in Cambridge, Massachusetts; the Lincoln Laboratory at MIT and the Space Telescope Science Institute in Baltimore. More than a dozen universities, research institutes and observatories around the world, including the University of Granada, participate in the mission.
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