The PLATO mission will monitor the brightness variations of more than 200,000 stars to look for planets which transit their host stars. TESS will also utilise the transit technique to focus on targeting bright host stars, to look for Earth-sized planets. It is thus not surprising when people end up thinking: why launch two space-based exoplanet spacecraft to look for Earth-sized planets?

The Transiting Exoplanet Survey Satellite (TESS) launched in April 2018 and has already started science operations. The two-year all-sky transit survey, like PLATO, will monitor more than 200,000 stars and be able to detect Earth-sized planets currently out of reach of ground based observatories.

With TESS being an all-sky survey, it won’t focus on any particular patch of sky for longer periods of time, but will instead move around. The exceptions being the more polar areas which have overlapping regions as shown in the figure below. The mission was designed this way: find the exoplanets orbiting the brightest stars all over the sky.

Owing to this mission goal, TESS will not be able to detect Earth-sized planets in the habitable zones of Sun-like stars. Why? To be able to detect a planet on a year long orbit (like Earth) one has to keep staring at the same patch of sky for at least a few years in order to see the brightness dips required to detect a planet using the transit technique. The combination of a whole-sky survey combined with the ability to detect planets with Earth-like orbits would far extend the proposed mission timelines.

The original Kepler mission did, however, stare and keep staring at the same patch of sky. This was because Kepler was a mission aimed at obtaining a statistical sample of planets so that the structure and diversity of planetary systems could be understood. A greater sample size was deemed more important than tuning the telescope to be sensitive to a few bright stars, so the original Kepler mission focused all it’s efforts on observing the same patch of sky avoiding stars which were too bright (such as close by Vega and Deneb). I use the words “original Kepler mission” as Kepler later ended up surveying different patches of the sky in what was named the K2 mission.

The data from the Kepler mission resulted in an amazing collection of planets displaying an immense diversity. Unfortunately, as most detected planets orbit faint stars, subsequent characterisation has been challenging. Even determining a planet’s mass through the radial velocity technique has often been impracticable as it would require a lots of telescope time to collect enough photons.

We live in the golden age of exoplanet science. With TESS we will be able to detect rocky planets which orbit close-by (and thus bright) M-dwarf stars. This is super exciting as most stars in our galaxy are M-dwarfs.

PLATO will detect and characterise Earth-size planets orbiting in the “habitable zone” of Sun-like stars. Furthermore it will accurately determine the physical parameters and ages of the host stars which are very important if we wish to properly characterise the bulk properties of the planets and to be able to understand their formation. Like TESS it will target Earth-size planets around bright stars and like Kepler, it will stare at patches of sky long enough to be sensitive to Earth-size planets on year long orbits around Sun-like stars.