How is TESS able to detect more planets in the sky than Kepler?

The new TESS mission covers almost the entire sky, compared to the small portion of sky observed by Kepler.

During its lifetime, Kepler observed only a few small (blue) fields of about 116 square degrees each on the sky: one in Cygnus and several along the ecliptic plane. TESS’s four cameras (yellow, orange, red, and violet) are designed to cover nearly the entire sky every two years. The newest telescope observes each of the 13 sectors, which measure 24° by 96° each, for 27.4 days before moving on to the next. This animation compares Kepler’s four years of observations with the first six years of sky data covered by TESS. Credit: Maps: Astronomy: Roen Kelly, from https://tess.mit.edu. Milky Way map: NASA/Goddard Space Flight Center Scientific Visualization Studio. Gaia DR2: ESA/Gaia/DPAC. Projection: G.Projector 3 — Map Projection Explorer. Animation: Darren Case.

How is TESS able to detect more planets in the sky than Kepler?

Doug Kaupa
Council Bluffs, Iowa

The Transiting Exoplanet Survey Satellite (TESS) has outperformed the now-retired Kepler mission in discovering planets and planet candidates, largely because the former’s survey area is significantly larger. It covers nearly the entire sky compared to the small patch of sky observed by Kepler. To maximize all-sky coverage, TESS observes about 1 million planet-detectable stars for about 27 days before moving on to another set of targets. Tens of millions of such stars have been observed since the mission began. Kepler, meanwhile, observed a smaller number of stars, but for a much longer and more sustained period of time, observing about 150,000 Sun-like stars over four consecutive years.

Basically, TESS has access to a significantly larger set of stars in which it can find planets. The TESS mission has also been in operation for longer than Kepler, lasting nearly six years (and counting), compared to Kepler's four.

The different observing strategies adopted by Kepler and TESS were chosen to support their specific science goals, and both missions have advanced our understanding of exoplanetary systems in unique ways. Kepler’s focused observations provided crucial insights into the demographics of small exoplanets with orbital periods of up to several years, as well as the architectures of multiplanet systems around Sun-like stars. Kepler also provided the best data set we have for understanding the prevalence of Earth-like exoplanets in our galaxy.

TESS complements Kepler by identifying exoplanets around a more diverse array of stars and their environments. Additionally, by focusing on detecting planets around bright stars across the sky, TESS is finding targets that are perfect for follow-up observations to measure planet masses and atmospheric compositions. Together, Kepler and TESS have made significant contributions to the discovery and understanding of exoplanets.

Michelle Kunimoto
Torres Postdoctoral Fellow, MIT Kavli Institute for Astrophysics and Space Research, Cambridge, Massachusetts