TESS discovers new worlds in a river of young stars

Using observations from NASA’s Transiting Exoplanet Survey Satellite (TESS), an international team of astronomers has discovered three hot worlds larger than Earth orbiting a much younger version of our sun called TOI 451. The system is located in the recently discovered Pisces-Eridanus Current, a collection of stars less than 3% older than our solar system and spanning a third of the sky.

The planets were discovered in TESS images taken between October and December 2018. Follow-up studies of TOI 451 and its planets include observations made in 2019 and 2020 using NASA’s Spitzer Space Telescope, which has since been retired, as well as many ground-based facilities. Archival of infrared data from NASA’s Near-Earth Object Wide-Field Infrared Survey Explorer (NEOWISE) satellite – collected between 2009 and 2011 under its previous name, WISE – suggests the system contains a cool disk of dust and rocky debris. Other observations show that TOI 451 likely has two distant star companions circling each other far beyond the planets.

“This system checks a lot of boxes for astronomers,” said Elisabeth Newton, an assistant professor of physics and astronomy at Dartmouth College in Hanover, New Hampshire, who led the study. “It is only 120 million years old and only 400 light years away, allowing for detailed observations of this young planetary system. And because there are three planets between two and four times the size of Earth, they are very promising targets for testing. theories of how a planet’s atmosphere evolves. “

A paper detailing the findings was published Jan. 14 in The Astronomical Journal

Stellar currents arise when the gravity of our Milky Way Galaxy tears apart star clusters or dwarf galaxies. The individual stars move along the cluster’s original orbit, forming an elongated group that gradually disintegrates.

In 2019, a team led by Stefan Meingast from the University of Vienna used data from the European Space Agency’s Gaia mission to discover the Pisces-Eridanus Current, named after the constellations with the greatest concentrations of stars. The current spans 14 constellations and is about 1,300 light-years long. However, the initially determined age for the stream was much older than we now think.

Later in 2019, researchers led by Jason Curtis of Columbia University in New York City analyzed TESS data from dozens of stream members. Younger stars spin faster than their older counterparts, and they usually have prominent star spots too – darker, cooler regions like sunspots. As these spots rotate in and out of our view, they can produce tiny variations in a star’s brightness that TESS can measure.

The TESS measurements revealed overwhelming evidence of starspots and rapid rotation between the stars of the current. Based on this result, Curtis and his colleagues found that the stream was only 120 million years old – comparable to the famous Pleiades cluster and eight times younger than previous estimates. The mass, youth and proximity of the Pisces-Eridanus stream make it an exciting foundational laboratory for studying the formation and evolution of stars and planets.

“Thanks to TESS’s near-complete coverage, measurements that could support a search for planets orbiting members of this stream were already available to us when the stream was identified,” said Jessie Christiansen, a co-author of the paper and deputy scientist. head at the NASA Exoplanet Archive, a facility for exploring worlds beyond our solar system, operated by Caltech in Pasadena, California. “TESS data will continue to allow us to push the boundaries of what we know about exoplanets and their systems in the years to come.”

The young star TOI 451, known to astronomers as CD-38 1467, is about 400 light-years away in the constellation Eridanus. It has 95% of the mass of our sun, but it is 12% smaller, slightly cooler and gives off 35% less energy. TOI 451 rotates every 5.1 days, which is more than five times faster than the sun.

TESS discovers new worlds by looking for transits, the light, regular dimmings that occur when a planet passes in front of its star from our perspective. Transits of all three planets are evident in the TESS data. Newton’s team got measurements from Spitzer that supported the TESS findings and helped rule out possible alternative explanations. Additional follow-up observations came from the Las Cumbres Observatory – a global telescope network headquartered in Goleta, California – and the Perth Exoplanet Survey Telescope in Australia.

Even the most distant planet from TOI 451 orbits three times closer than Mercury ever approaches the sun, so all of these worlds are quite hot and inhospitable to life as we know it. Temperature estimates range from about 2,200 degrees Fahrenheit (1,200 degrees Celsius) for the inner planet to about 840 F (450 C) for the outermost.

TOI 451 b orbits every 1.9 days, is about 1.9 times the size of the Earth and its estimated mass varies from two to 12 times the Earth. The next planet outside, TOI 451 c, orbits every 9.2 days, is about three times the size of the Earth, and between three and 16 times the mass of the Earth. The farthest and largest world, TOI 451 d, orbits the star every 16 days, is four times the size of our planet and weighs between four and 19 Earth’s masses.

Astronomers expect that planets as big as this one will retain much of their atmosphere despite the intense heat of their nearby star. Different theories of how atmospheres will evolve by the time a planetary system reaches TOI 451 age predict a wide variety of properties. Observing starlight passing through the atmospheres of these planets offers the opportunity to study this phase of development and could help limit current models.

“By measuring starlight entering a planet’s atmosphere at different wavelengths, we can infer the chemical composition and presence of clouds or high-altitude fog,” said Elisa Quintana, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. . “The planets of TOI 451 provide excellent targets for such studies with Hubble and the upcoming James Webb Space Telescope.”

Observations from WISE show that the system is unusually bright in infrared light, invisible to human eyes, at wavelengths of 12 and 24 microns. This suggests the presence of a debris disk, where rocky asteroid-like bodies collide and grind themselves to dust. Although Newton and her team cannot determine the size of the disk, they see it as a diffuse ring of rock and dust about as far from the star as Jupiter from our sun.

The researchers also examined a faint neighboring star that is about two pixels away from TOI 451 in TESS images. Based on Gaia data, Newton’s team determined that this star is a gravity-bound companion that far from TOI 451. is that it takes 27 days to get there. In fact, the researchers think the companion is likely a binary system of two M-type dwarf stars, each containing about 45% of the Sun’s mass and emitting only 2% of its energy.