NASA’s Transiting Exoplanet Survey Satellite (TESS) has hundreds of “objects of interest” waiting to be confirmed as planets in the data from the space telescope’s four cameras. These three were the first confirmed TESS discoveries, identified last year during its first three months of observing. By the time the mission is done, TESS’s wide-field cameras will have covered the whole sky in search of transiting exoplanets around 200,000 of the nearest (and brightest) stars. (NASA / MIT / TESS)
The newest space telescope in the sky — NASA’s Transiting Exoplanet Survey Satellite, TESS — has been searching for exoplanets for less than a year, but already it has quite a collection to its name.
The TESS mission is to find relatively nearby planets orbiting bright and stable suns, and so expectations were high from the onset about the discovery of important new planets and solar systems. At a meeting this week at the Massachusetts Institute of Technology devoted to TESS results, principal investigator George Ricker pronounced the early verdict.
The space telescope, he said, “has far exceeded our most optimistic hopes.” The count is up to 21 new planets and 850 additional candidate worlds waiting to be confirmed.
Equally or perhaps more important is that the planets and solar systems being discovered promise important results. They have not yet included any Earth-sized rocky planet in a sun’s habitable zone — what is generally considered the most likely, though hardly the only, kind of planet to harbor life — but they did include planets that offer a great deal when it comes to atmospheres and how they can be investigated.
This infographic illustrates key features of the TOI 270 system, located about 73 light-years away in the southern constellation Pictor. The three known planets were discovered by NASA’s Transiting Exoplanet Survey Satellite through periodic dips in starlight caused by each orbiting world. Insets show information about the planets, including their relative sizes, and how they compare to Earth. Temperatures given for TOI 270’s planets are equilibrium temperatures, (NASA’s Goddard Space Flight Center/Scott Wiessinger)
One of the newest three-planet system is called TOI-270, and it’s about 75 light years from Earth. The star at the center of the system is a red dwarf, a bit less than half the size of the sun.
Despite its small size, it’s brighter than most of the nearby stars we know host planets. And it’s stable, making its solar system especially valuable. When variations in the star’s light are minimal, and they’re less likely to get in the way of trying to pick up subtle changes caused by its orbiting planets.
While none of the three planets are likely habitable, more planets may yet be found farther out in the star system, orbiting in more habitable orbits. A paper describing the system was published in the journal Nature Astronomy.
“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,” said lead researcher Maximilian Günther, a Torres Postdoctoral Fellow at the (MIT) Kavli Institute for Astrophysics and Space Research in Cambridge.
Compare and contrast worlds in the TOI 270 system with these illustrations of each planet. Temperatures given for TOI 270 planets are equilibrium temperatures, calculated without taking into account the warming effects of any possible atmospheres.
(NASA’s Goddard Space Flight Center)
“This star is quiet and very close to us, and therefore much brighter than the host stars of comparable systems. 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.”
This is essential both in terms of understand the particular planet, and in developing methods for reading the atmospheres of exoplanets more generally. Those readings will hopefully some day tell researchers that they have found a planet with an atmosphere out of chemical balance in ways that could only be the result of biology.
The authors estimate that the James Webb Space Telescope, now scheduled to launch in 2021, will eventually have a view of the system for over half the year, and it should be able to pick out the atmospheric signals for both planets.
As explained in a NASA release, the innermost planet, TOI 270 b, is likely a rocky world about 25% larger than Earth. It orbits the star every 3.4 days at a distance about 13 times closer than Mercury orbits the sun. Based on statistical studies of known exoplanets of similar size, the science team estimates TOI 270 b has a mass around 1.9 times greater than Earth’s.
Maximilian Guenther is a Torres Fellow at the MIT, where he work closely with the TESS team.
Due to its proximity to the star, planet b is an scalding-hot world. Its equilibrium temperature — that is, the temperature based only on energy it receives from the star, which ignores additional warming effects from a possible atmosphere — is around 490 degrees Fahrenheit (254 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. Although only about half its size, both may be similar to Neptune in our solar system, with compositions dominated by gases rather than rock. They likely weigh around 7 and 5 times Earth’s mass, respectively.
All of the planets are expected to be tidally locked to the star, which means they only rotate once every orbit and keep the same side facing the star at all times, just as the Moon does in its orbit around Earth.
Planet c and d might best be described as mini-Neptunes, a type of planet not seen in our own solar system. The researchers hope further exploration of TOI 270 may help explain how two of these mini-Neptunes formed alongside a nearly Earth-size world.
“An interesting aspect of this system is that its planets straddle a well-established gap in known planetary sizes,” said co-author Fran Pozuelos, a postdoctoral researcher at the University of Liège in Belgium.
“It is uncommon for planets to have sizes between 1.5 and two times that of Earth for reasons likely related to the way planets form, but this is still a highly controversial topic. TOI 270 is an excellent laboratory for studying the margins of this gap and will help us better understand how planetary systems form and evolve.”
Only 31 light-years away from Earth, the exoplanet GJ 357 d catches light from its host star GJ 357, in this artistic rendering.
And then there’s the planetary system of GJ 357.
The newly discovered planets orbit an M-type dwarf about one-third the sun’s mass and size and about 40% cooler that our star. The system is located 31 light-years away, which makes it a relatively close neighbor.
In February, TESS cameras caught the star dimming slightly every 3.9 days, revealing the presence of a transiting exoplanet that passes across the face of its star during every orbit and briefly dims the star’s light. That discovery led to the finding of two more planets around the star, including one that may be quite promising.
Rafael Luque is a researcher at the Institute of Astrophysics of the Canary Islands — where several important telescopes are located.
“In a way, these planets were hiding in measurements made at numerous observatories over many years,” said Rafael Luque, a doctoral student at the Institute of Astrophysics of the Canary Islands (IAC) on Tenerife, who led the discovery team. “It took TESS to point us to an interesting star where we could uncover them.”
But while researchers were looking at ground-based data to confirm the existence of the hot Earth, they uncovered two additional worlds. The farthest-known planet, named GJ 357 d, is the one that really caught their attention.
“GJ 357 d is located within the outer edge of its star’s habitable zone, where it receives about the same amount of stellar energy from its star as Mars does from the sun,” said co-author Diana Kossakowski at the Max Planck Institute for Astronomy in Heidelberg, Germany.
“If the planet has a dense atmosphere, which will take future studies to determine, it could trap enough heat to warm the planet and allow liquid water on its surface.”
This GJ 357 system illustrates well how exoplanet discoveries are gathered, confirmed and then interpreted.
Transit data are rich with information. By measuring the depth of the dip in brightness and knowing the size of the star, scientists can determine the size or radius of the planet. The orbital period of the planet can be determined by measuring the elapsed time between transits. Once the orbital period is known, Kepler’s Third Law of Planetary Motion can be applied to determine the average distance of the planet from its stars. (NASA Ames)
A planet orbiting GJ 357 was first identified via the transit method by TESS. Then it was confirmed using the ground-based radial velocity data collected from numerous ground-based telescopes over the years. That data was recoded and re-interpeted (with the assistance of the Carnegie Institution’s Paul Butler (who was part of the team that confirmed the detection of the first exoplanet in 1995) and the additional two planets were identified.
Then the information was put through models by an interdisciplinary team and this announcement was the result:
“An international team of astronomers… has characterized the first potentially habitable world outside of our own solar system.” The paper appeared in the journal Astrophysical Journal Letters.
This artist’s illustration demonstrates the “wobble,” or radial velocity, technique for finding planets. The planet-detection technique relies on the fact that stars wobble back and forth as their planets circle around, tugging on them with their gravity. As a star moves toward us, the color of its light shifts to shorter, or bluer, wavelengths. As the star heads away, its light stretches into longer, or redder, wavelengths. The same principle, called the Doppler effect, causes sound from a speeding train to lower in pitch as it passes by. By measuring changes in the wavelength of light from a star, astronomers can track changes in the star’s velocity that arise from circling planets. By measuring the speed of the star and the period of the wobble, they can determine the mass and distance of the unseen planet, respectively. (NASA)
“This is exciting, as this is humanity’s first nearby super-Earth that could harbor life – uncovered with help from TESS, our small, mighty mission with a huge reach,” said Lisa Kaltenegger, associate professor of astronomy, director of Cornell’s Carl Sagan Institute and a member of the TESS science team.
Lisa Kaltenegger is the Director of the Carl Sagan Institute at Cornell and Associate Professor in Astronomy. Her research focuses on exploring new worlds orbiting other stars, especially rocky planets and super-Earths and their atmospheres in the habitable zone. She is an expert in modeling potential habitable worlds and their detectable spectral fingerprint. (Cornell University.)
The exoplanet is more massive than our planet, and Kaltenegger said the discovery will provide insight into Earth’s heavyweight planetary cousins. “With a thick atmosphere, the planet GJ 357 d could maintain liquid water on its surface like Earth, and we could pick out signs of life with telescopes that will soon be online,” she said.
How did Kaltenegger and her colleagues get to that conclusion?
The planet receives little more than a third of the radiation that Earth receives, making it similar to Mars. If the planet released gases present since its formation at a rate similar to Earth, the surface temperature would remain below freezing.
But as their paper concludes:
“Geological active worlds, like our Earth, are expected to build up CO2 concentrations due to the feedback of the carbonate-silicate cycle. We model atmospheres (with and without oxygen) as three examples, where we increase CO2 concentration so that the planet’s average surface temperature is above freezing.”
“The sample reflection, emission and transmission spectra show features of a wide range of chemicals — water, carbon dioxide, methane, ozone and oxygen for Earth-like atmospheres from the Visible to Infrared wavelength — which would indicate habitability for observations with upcoming telescopes.”
This is how the exoplanet drama works. Each significant discovery makes possible a future discovery, then additional hypotheses are put forward that often need new and more powerful viewing telescopes to prove or disprove. There are many goals in this enterprise, but the big one is clearly the discovery of clear signs of life far beyond Earth.
