Data is coming in from the Transiting Exoplanet Survey Satellite, known as TESS for short. It is the most powerful telescope ever deployed to look for planets orbiting other stars. Over two years, TESS can cover all 360 degrees of sky visible from Earth’s orbit. Our previous satellite called Kepler could only scan a small segment of the sky. Already Tess has identified over 300 probable exoplanets including one named HD 21749b which has the lowest known temperature for a planet orbiting a bright nearby star. (“Nearby” being 53 light-years away.)
The problem with this is that what astronomers consider “cool” is not cool from our standpoint. The surface temperature of HD 21749b is 150 degrees Celsius, which is way too hot for liquid water. (Water boils at 100 degrees Celsius.) A year on that planet equals 36 Earth days as it makes a complete orbit around its star. Most of the other exoplanets found at this time are vastly hotter than HD 21749b.
Astronomers have found other planetary systems, but they again have properties that would preclude any kind of life. Some of them have a planetary density equal to that of pure water. Some have orbits that are highly eccentric. Pi Mensae b, for example, has an orbit that varies widely. Its closest distance to its star approximately equals the distance from Earth to our Sun. The longest distance is similar to Jupiter’s distance from the Sun.
In 1999 astronomers detected the first exoplanet–a planet in another solar system. The number of planets detected orbiting around stars other than our Sun has grown to more than 3,500 today. There are billions of stars in our Milky Way Galaxy so searching for other worlds is just getting started.
NASA’s main tool for finding exoplanets has been the Kepler space telescope. The method of detecting those planets is watching for occlusions. If there is a planet orbiting a star, it will sometimes pass in front of that star from our viewpoint causing an occlusion or mini-eclipse. The planets are too small for us to see, but we can see a small dip in the light coming from the star. If the dip comes on a regular interval that means it might be an orbiting planet. The amount of the dip in light level indicates the size of the planet in relation to its star. Using this method of detection, astronomers have compiled a catalog of detected planets.
As I said, until now the Kepler telescope has been the method for finding most of these planets, but it will soon end its life. However, 2018 will be the beginning of new opportunities to look for exoplanets because of two new satellite-based observatories. Very soon NASA will launch TESS. That stands for Transiting Exoplanet Survey Satellite. By the end of the year, the European Space Agency (ESA) will launch CHEOPS or Characterising Exoplanet Satellite. (Aren’t you glad we have acronyms.)
The Kepler telescope was very good at searching for other worlds, but only in a small area of the sky. TESS will take a much wider view with the hope of finding many more. For obvious reasons, so far most of the planets detected are giant planets. TESS will be targeting bright stars in the hope of finding smaller planets that more closely resemble Earth. Astronomers will be able to target TESS more precisely toward selected stars.