TESS space telescope will find thousands of planets, but astronomers are looking for a small number of planets | Science

The most recent NASA planetary hunter probably found his first planetand a few days later, his second. Astronomers await independent confirmation from other telescopes, but the glut of new planets could be realized through a list of candidates – an impressive feature for a space telescope that took its first scientific image less than two months ago .

The exoplanet surveillance satellite in transit (TESS) was launched in April this year and spent a few months reaching its final orbit. TESS will conduct a two-year investigation to analyze about 85% of the sky. The observatory in orbit should spot thousands of exoplanets around the nearest and brightest stars – perfect candidates for further studies using larger telescopes.

The first two planets discovered by TESS are strange worlds. It looks like a miniature Neptune, but is much closer to its star, making it hotter and more volatile than the great blue planet of our solar system. The other is a world the size of Earth locked in a hug so narrow that its orbit completes an orbit in just 11 hours, making the surface conditions particularly infernal.

But thousands of exoplanets that TESS will reveal that scientists are primarily interested in a fraction of 1%. Natalia Guerrero, the deputy director of the TESS Objects of Interest (TOI) at MIT, says the spacecraft is really looking for about 50 planets, each less than four times the Earth's mass and in the living area of their star.

"We hope we will find more than that, but that's what we expect," says Guerrero.

While the complete catalog of exoplanets will help scientists learn about the distribution of planets in the galaxy, the handle that Guerrero hopes to find most are ideal targets in the search for another life-carrying world. A planet of less than four land masses is probably rocky, for example, and if it also orbits at a good distance from its star (which depends on the size and temperature of the star), it could support surface water liquids. A number of other factors influence the actual existence of life on a planet – the abundance of organic compounds, for example – but, given what we know of the Earth, the Rocky planets with water seem to be the best place to start.

In addition, the next generation of large telescopes will be able to study these planets in an impossible way with current telescopes. TESS finds planets via the transit method, which looks for the hollows in the starlight caused by a planet moving in front. The space telescope's sophisticated light sensors "can detect the amount of light a butterfly blocks when it crosses the full moon," says Guerrero. Therefore, every planet that TESS discovers will be the one that passes its star from our point of view, which represents an opportunity to find targets for the next big step in the search for a world similar to the Earth: measuring the atmospheres .

To understand the composition of an exoplanet atmosphere, you need an instrument called a spectrometer, capable of analyzing the chemical signature of celestial objects. When a planet passes in front of its star, part of the starlight passes through the atmosphere and some wavelengths are filtered by the presence of specific elements. At the moment, with something like the Hubble Space Telescope, such measurements can only be made for planets the size of Jupiter or larger. But giant space telescopes, such as the upcoming James Webb Space Telescope, a tennis-sized monstrosity planned for launch in 2021, will isolate the light passing through the atmosphere of a smaller planet. . . With the right telescope and the right conditions, an atmosphere could be revealed, and a world we know to be the size of the Earth could really become Earth-like if it has an atmosphere like ours.

TESS is looking for relatively close planets, about 330 light-years away, scattered in the northern and southern hemispheres (the space telescope is currently looking south and will turn north next year). This approach is different from NASA's other space telescope, Kepler, which has been looking at the same star field for about four years. While Kepler can collect enough light to find planets up to 3,000 light-years away, the new survey mission with TESS will find nearby planets in all places of the sky.

Abel Mendez, director of the Global Habitability Lab at the University of Puerto Rico in Arecibo, maintains a catalog of potentially habitable planets found so far. At the moment, there are a total of 55 planets on the list, with 22 of the size of the Earth, one of the size of Mars and 32 of the "Super-Earth" which are about twice the size of the planet. size of the Earth. (These are optimistic figures – according to the strictest set of criteria, only 14 planets make the "livable" cut.)

Mendez estimates that TESS will produce "dozens" of new habitable exoplanets and that the telescope will also allow his team to study some of these distant worlds. Of the few twelve habitable planets known, only four are in the field of view of Arecibo. "TESS will provide many more targets for monitoring observations by Arecibo and many more telescopes around the world," said Mendez.

In addition to probing distant atmospheres, tracking observations made with telescopes around the world can teach us a surprising amount on distant planets. Take Pi Mensae c, for example, the first planet discovered by TESS. The world revolves around the Pi Mensae star about 60 light-years away, or 350 trillion miles. It completes an orbit in just 6.27 days and has a radius twice as large as the Earth – two parameters that TESS can measure as the planet passes in front of its host star.

But TESS can not measure another crucial piece of information: mass.

The planet Pi Mensae is nearly five times the mass of the Earth. To calculate this figure, astronomers had to go back to the old data of the star. Pi Mensae, or HD 39091, was already known to have a giant planet – ten times the mass of Jupiter. This planet was discovered in 2001 by the Anglo-Australian Planet Search (one of the first groups of exoplanet hunters) through a process called radial velocity. The radial velocity method examines perturbations in the movement of a star, which may indicate that a planet is pulling on the star with its gravity. The technique is primarily useful for finding large planets, but once discovered, radial velocity measurements can be used to calculate mass.

"While the [new] the signal from the planet is definitely present in the data that already existed, not very big because the planet is so small, "says Jennifer Burt, postdoctoral Torres at the Institute of Astrophysics and Space Research MIT Kavli. first planet TESS.

After determining the mass of Pi Mensae c, astronomers realized that the planet is probably closer to an extremely small variant of Neptune than a very large variant of the Earth. But just days after announcing their first planet, the TESS team had already found its second – and it's really strange.

The planet revolves around the star LHS 3844, which is a small, cool star called red dwarf. These small, fresh stars represent the most abundant star type of the universe. Many red dwarfs have planets that orbit in a few days, but the newly discovered planet around LHS 3844 completes its orbit in just 11 hours. A week on Earth represents 15 "years" on this planet, placing it in a small class of ultra-short period planets.

Initially, TESS discoveries will be directed to these types of planets – repeated transits on the star mean they are easier to detect. With dozens of strong candidates still on the TESS list for this sector, there could be a lot of discoveries around the corner. Such collateral planets will pile up during the mission while waiting to discover other Earth-like worlds that orbit around their stars in a few hundred days.

But as Burt points out, even inhospitable planets paint a brilliant story of the night sky. The star Pi Mensae, now with two known planets, is bright enough to see at the naked eye. When the initial two-year mission of TESS is complete – and the team is already planning a mission expansion – we should understand which nearby stars have their own orbiting worlds.

"By the time TESS finishes its mission, you should be able to go out into your garden, point a star and know that there are planets around you," Burt said. Maybe one of these planets will look even like ours.