UW Astrobiology Faculty Member Dr. Eric Agol is a Professor of Astronomy and a TESS Science Team Member. Here he uses his expertise and insights to describes the TESS advantage in the search for other worlds that could harbor life!
By Dr. Eric Agol
Professor Astronomy & Astrobiology
Anticipation has been building for the discovery of new exoplanets with the recent successful launch of the TESS spacecraft on April 18th. TESS uses the same approach as Kepler in searching for periodic transits of stars by planets whose orbits pass in front of the star. However, Kepler surveyed only 100 square degrees, while TESS will survey the whole sky (nearly), an area about 400 times greater than the area covered by Kepler. In order to carry this out, TESS is designed with four wide-field cameras which each view about five times the area of Kepler, for a total of about twenty times the viewing area. However, the trade-off with the field of view is that TESS has a much smaller photon collecting area: each TESS camera looks through optics which are similar to a telephoto lens. TESS will also survey each portion of the sky for only four weeks at a time (although with longer duration of coverage near the poles), with 13 survey fields of the Northern sky for a year, followed by the Southern sky. So for a given star, TESS is less sensitive than Kepler, and will find planets with larger transit depths in shorter orbital periods, However, TESS’ full-sky coverage will enable an extensive survey of planets around the brightest stars, which are more likely to be closer to us, in comparison to the Kepler’s small area, deep survey, that found planets around stars that were much farther away.
These brighter host stars that TESS will find planets around are more amenable to follow-up with ground-based Doppler/radial-velocity measurements, and will also be advantageous when searching for atmospheric spectral transmission features with the James Webb Space Telescope. In addition, TESS will include a larger fraction of M dwarf stars, whose smaller dimensions giving the satellite some chance of finding rocky, temperate planets in the Habitable Zone of their parent stars.
An additional difference between TESS and Kepler is that TESS is a satellite (orbiting Earth), while Kepler is a spacecraft (orbiting the Sun). This means that to avoid the Earth, TESS has been given an elongated orbit which is resonant with the Moon (to avoid a collision). TESS spends most of its orbit (near apogee) away from the Earth where it can survey the sky unobstructed for four weeks before needing to re-orient itself to avoid the Sun. During the short perigee passage (closest approach), TESS gets close enough to Earth to transmit data it has collected with higher bandwidth. This proximity to Earth enables a higher data collection rate than Kepler, meaning that TESS can download its entire field of view with a cadence of 30 minutes (and a smaller portion with targeted stars using a 2-minute cadence), enabling a broader search for variable phenomena than was allowed by the Kepler spacecraft. As with most of astronomy, when new capabilities are opened, usually the most interesting discoveries are unanticipated. It falls on us to search the TESS data for whatever secrets the Universe is willing to divulge.”
Liftoff of the SpaceX Falcon 9 rocket from Cape Canaveral Air Force Station carrying TESS into a high earth orbit