The Transiting Exoplanet Survey Satellite (TESS) spacecraft, NASA’s newest eye in the sky searching for worlds around other stars, has scored a hat trick of new discoveries near our own Solar System. The three previously unknown worlds discovered by TESS are seen orbiting the star UCAC4 191–004642, 73 light years from Earth.
The planetary system, with its parent star, now designated TOI (TESS Object of Interest) 270, contains at least three worlds. One of these is slightly larger than the Earth, while two are sub-Neptunes, a class of planet not found in our Solar System. The star is cool and faint, roughly 40 percent smaller and less massive than the Sun. It’s surface temperature is roughly two-thirds the temperature of our own stellar companion.
“This system is exactly what TESS was designed to find — small, temperate planets that pass, or transit, in front of an inactive host star, one lacking excessive stellar activity, such as flares. This star is quiet and very close to us, and therefore much brighter than the host stars of comparable systems,” said Maximilian Günther of the Massachusetts Institute of Technology.
Getting to Know the Neighbors
The innermost planet, TOI 270 b, is roughly 25 percent larger, and 90 percent more massive, than our home world. Like the Earth, TOI 270 b is a rocky world. It orbits its parent star once every 3.4 days, at a distance less than eight percent as far as the distance between Mercury and the Sun, making this is a scorching hot world. Even without any warming from any atmosphere, this planet would have a temperature of 254 degrees Celsius (490 Fahrenheit).
The next planet out from the star is TOI 270 c, 2.4 times larger than the Earth, and seven times more massive. This world orbits its star once every 5.7 days. At this distance, without an atmosphere, this planet would have a surface temperature of 150 degrees C (300 F), a typical temperature of a household oven.
The second sub-Neptune found by astronomers around this world is TOI 270 d. Orbiting around its parent star once every 11.4 days, this newly-discovered planet is possesses five times the mass of Earth, packed into a sphere 2.1 times larger than our home world. The surface of this world, if an atmosphere were not present, would hover around 67 degrees Celsius (150 F). At this temperature, extremophiles, the hardiest forms of life on the Earth, might be able to survive, and bodies of water could exist. However, any atmosphere would add to these temperatures, making life on that world unlikely.
Each of the three planets in this system are thought to be tidally locked to their Sun. This would mean they each have one side facing the parent star, in much the same way one side of the Moon always faces the Earth.
The TOI 270 solar system is found in the constellation Pictor. The worlds found in that planetary system are so close to their star that the orbits of the three worlds could fit within the family of moons accompanying Jupiter.
Our own solar system is composed of four small rocky planets (Mercury, Venus, Earth, and Mars), two gas giants (Jupiter and Saturn) and a pair of ice giants (Uranus and Neptune). In other solar systems, the most common type of planet are sub-Neptunes, worlds larger than the Earth, but smaller than Neptune.
It’s Cool to be this Hot!
“We’re all aware of the need for salt on a hot and arid planet like this, Professor, but it’s a mystery, and I don’t like mysteries. They give me a bellyache and I got a beauty right now.”
- James Kirk, Star Trek: TOS, The Man Trap
Of the three worlds, the outermost world may be the most intriguing. Temperatures not far from temperate worlds make TOI 270 d the most Earth-like world in the system, and a rarity among sub-Neptunes. However, if this planet has a substantial atmosphere, it may be too warm for water to pool on its surface.
Why our own solar system is devoid of worlds in this class is one of the great new mysteries in astronomy. Discoveries in science often lead to new questions, and one conundrum poised by this new finding from TESS is how these sub-Neptunes formed so close to smaller, rocky worlds.
“With extended follow-up observations, we’ll soon be able to determine the make-up of these worlds, establish if atmospheres are present and what gases they contain, and more,” Günther stated.
Additional research on the system could reveal additional planets in the TOI 270 solar system. Planets in that system, revolving further from their Sun than the orbit of TOI 270 d, might be cool enough to hold onto liquid water, and possibly develop life.
One of the most-intriguing aspects of this system is how this solar system contains both a small rocky world and gaseous sub-Neptunes. The star at the center of the TOI 270 system is both close to us and also fairly quiet, with few flares. These characteristics allow astronomers a chance to make easy measurements of dips in brightness from the star which result from orbiting planets.
“We’ve found very few planets like this in the habitable zone, and many fewer around a quiet star, so this is rare. We don’t have a planet quite like this in our solar system,” Stephen Kane, associate professor of planetary astrophysics at the University of California Riverside, explains.
Putting Planet Hunting to the TESS
The TESS observatory was launched into space in April 2018, aboard a Falcon 9 rocket. In its first year in space, TESS has found more than 20 exoplanets orbiting alien stars, as well as 900 unconfirmed finds.
“One of the primary goals of exoplanetary science is to detect small, temperate planets passing (transiting) in front of bright and quiet host stars. This enables the characterization of planetary sizes, orbits, bulk compositions, atmospheres and formation histories,” researchers wrote in a journal article in Nature Astronomy describing the discoveries around the star TOI 270.
TESS finds new worlds by watching stars, looking for dips in brightness resulting from the passing of planets in front of that star, as seen from Earth.
When the James Webb Space Telescope launches (hopefully in 2021), that instrument will be able to examine atmospheres of these worlds for signs of oxygen, carbon monoxide, and hydrogen. Such observations could help us better understand the nature of the planets around TOI 270 and other sub-Neptunes.
Currently, we know of more than 4,000 exoplanets orbiting alien stars. The TESS observatory will study the entire sky, examining 200,000 of the brightest stars for planets unknown to astronomers.
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