EXOPLANETS | Space instruments & demonstrators

EXOPLANETS | Space instruments & demonstrators

PLATO is the third medium class mission (M3) in ESA’s Cosmic Vision program. The space telescope will peer at thousands of stars for several years to find the tiny dips in intensity (0.01%) when a planet passes in front. By measuring the depth of the transit and the duration between transits, the size of the planet and its distance from the host star can be obtained.

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Artist's Impression of exoplanets, some with moons, and their orbits around a star. Credit: DLR

Because of the huge sample provided by PLATO (PLAnetary Transits and Oscillations of stars) and its ability to stare at the stars for years, astronomers will be able to find also small planets with long periods. This enables us to find Earth sized planets in the so-called ‘habitable zone’ around stars like the Sun.

The huge sample provided by PLATO will allow us to perform detailed statistics on the architecture of planetary systems. Ever since the discovery of the first planet around another star, astronomers have found that by far not every planetary system has a buildup comparable to our own Solar System.

The question is, how common (or rare) is our Solar System. The answer to this question can hold important clues about if and where we could find life beyond our Solar System. Also, it will help us put the ideas about how planetary systems form and evolve to the test.

PLATO was selected for the M3 slot in 2014 and is expected to be launched in 2026.

Science

Foremost, PLATO will provide statistical information on the architecture of planetary systems. However, since it will also look at the brightest stars in the sky, these planets can be followed up from the ground to provide detailed information on the bulk density of the planets. This tells us if the planet is, for example, mainly rock (like the Earth), mainly gas (like Jupiter), or very icy (like Pluto). Even more detailed information about the specific makeup of the planet can be obtained from this.

The detailed light curves provided by PLATO also provide information on the emission of thermal radiation from the planet, and the reflection of starlight by clouds in the atmosphere. Because we follow the planet through all its phases (from night-side to day-side) we can make a map of the distribution of heat and clouds over the day- and night-side for the brightest planets. This contains important information on currently highly unconstraint processes like dynamical mixing and cloud formation in the planet atmospheres.

Detecting an Earth sized planet in the habitable zone around a solar type star requires patience. When seen from afar in the right constellation, the Earth transits the Sun once per year. Therefore, PLATO needs to stare at the stars for a few years to detect at least two transits of these potentially habitable planets.

Besides detecting the planet transits, we can also obtain the seismic activity of the host star from the detailed lightcurve. The ‘astroseismology’ that we can do with this information provides us with details of the planet host stars like mass, radius and age. Knowing the age of the host star allows us to put the planetary statistics into an evolutionary sequence, and thereby constrain the evolution of planetary systems.

Technology

The PLATO satellite will consist of 26 optical refractive telescopes, each with a diameter of 20 cm. There are 24 ‘normal’ telescopes that take an image every 25 seconds, and 2 ‘fast’ telescopes for tracking (image every 2.5 seconds). Each of these telescopes have a slightly different, but mostly overlapping, field of view. Thereby PLATO can observe a large part of the sky simultaneously. Spreading the light over more telescopes  is crucial to improve the signal to noise ratio, and to observe as many bright stars as possible, allowing detailed follow-up observations with ground based telescopes.

The PLATO science requires extreme stability of the telescopes, so that observed variations in the light curves are only due to planetary transitions, and not to instrumental artefacts. We need to know exactly how each of the telescopes behaves. To this end, SRON is building a space simulator to test and characterize the telescopes under simulated space conditions. This requires vacuum and cryogenic temperatures, and a light source that can mimic a distant star.

SRON will do the cryogenic vacuum validation and characterization of 8 of the 24 normal cameras. Two other institutes in Europe, in France and Spain, will perform these tests on the remaining telescopes.



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