NASA & # 39; s Transiting Exoplanet Survey Satellite, TESS, has discovered its first Earth-sized exoplanet. The planet, called HD 21749c, is the smallest world outside of our solar system that TESS has identified so far.
In a newspaper published today in the magazine Astrophysical Journal Letters, a team of astronomers led by MIT reports that the new planet revolves around the star HD 21749 – a very near star, just 52 light years away from Earth. The star also houses a second planet – HD 21749b – a warm "sub-Neptune" with a longer orbit of 36 days, which the team has previously reported and now gives more details in the current article.
The new planet the size of a planet is probably a rocky but uninhabitable world, because it orbits its star in just 7.8 days – a relatively tight orbit that would generate surface temperatures of up to 800 degrees Fahrenheit on the planet.
The discovery of this earthly world is nevertheless exciting, because it shows the ability of TESS to select small planets around nearby stars. In the near future, the TESS team expects the probe to reveal even colder planets, with conditions that are more suitable for life.
"For stars that are very close and very bright, we expected to find a few dozen Earth-sized planets," said lead author and TESS member Diana Dragomir, a postdoc at the Kavli Institute for Astrophysics and Space Research at MIT. "And here we are – this would be our first, and it is a milestone for TESS. It sets the path for finding smaller planets around even smaller stars, and those planets may be habitable."
TESS has been hunting for planets outside our solar system since its launch on April 18, 2018. The satellite is a NASA Astrophysics Explorer mission led and controlled by MIT, and is designed to observe almost the entire sky, in overlapping, monthly long patches, or "sectors," revolving around the earth. While circling around our own planet, TESS points the four cameras out to follow the closest, brightest stars in the sky, looking for periodic breaks in the starlight that may indicate the presence of an exoplanet while these for his host star.
During its two-year mission, TESS wants to identify at least 50 small, rocky planets for the astronomical community, along with estimates of their masses. To date, the mission has discovered 10 planets smaller than Neptune, four of their estimated masses, including π Men b, a planet twice the size of the Earth, orbiting the six-star star days; LHS 3844b, a hot, rocky world that is slightly larger than the earth and circles its star in a sizzling 11 hours; and TOI 125b and c – two "sub-Neptunes" orbiting the planet in the same orbit, both within about a week. All four of these planets were identified on the basis of data obtained during the first two observation sectors of TESS – a good indication, the team writes in its article that "there are many more to be found".
Dragomir chose this newest planet on Earth from the first four sectors of TESS observations. When these data became available, in the form of light curves, or starlight intensities, they entered them into a software code to search for interesting, periodic signals. The code first identified a possible transit that the team later confirmed as the warm sub-Neptune they announced earlier this year.
As usual with small planets, where there is one, there are probably more, and Dragomir and her colleagues decided to reread the same observations to see if they could hide other small worlds that were hidden in the data.
"We know that these planets often occur in families," says Dragomir. "So we searched all the data again and this little signal came forward."
The team discovered a small dip in the light of HD 21749, which occurred every 7.8 days. Finally, the researchers identified 11 of such periodic declines, or transits, and determined that the star's light was temporarily blocked by an Earth-sized planet.
Although this is the first planet on Earth discovered by TESS, other Earth-sized exoplanets have been discovered in the past, mainly by NASA's Kepler Space Telescope, a retired telescope that has monitored more than 530,000 stars. Eventually, the Kepler mission discovered 2,662 planets, many of which were Earth-sized, and a handful of them were supposed to be within the habitable zone of their star – where a balance of circumstances might be appropriate to host life.
However, Kepler has observed stars that are many miles further than the stars followed by TESS. That is why Dragomir says that following one of Kepler's far-flung full-size planets is much more difficult than studying planets around the much closer, brighter stars of TESS.
"Because TESS follows stars that are much closer and brighter, we can measure the mass of this planet in the very near future, while that was not possible for Kepler & # 39; s planets on Earth," says Dragomir. "So this new TESS discovery could lead to the first mass measurement of a planet on Earth. And we are excited about what that mass could be. Will it be the mass of the Earth? Or heavier? We don't really know. & # 39;