Europe's Cheops telescope launches to study far-off worlds

The European Cheops space telescope has launched to study planets outside our Solar System.

The observatory will follow up the discoveries of previous missions, endeavouring to reveal fresh insights on the nature of distant worlds: What are they made of? How did they form? And how have they changed through time?

The telescope is being taken into orbit on a Russian Soyuz rocket that set off from French Guiana at 08:54 GMT.

The journey should take 145 minutes.

Cheops (short for CHaracterising ExOPlanet Satellite) is a joint endeavour of 11 member states of the European Space Agency (Esa), with Switzerland in the lead.

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  What's significant about this mission?

  Some 4,500 planets have been discovered since the late 1990s using a variety of techniques. But there is a feeling now that the science has to move beyond just detection; beyond just counting planets. We need to profile the objects in a more sophisticated way. Do they have atmospheres and how thick are they? What kind of clouds? Do they possess oceans on their surface? Do they have rings and moons? Cheops ought to be able to address such questions just from looking for these tiny dips in light during a transit.

  The mission has been given a list of 400-500 targets to look at over the next 3.5 years. Most of these worlds will be in the size range between Earth and Neptune, sometimes called "super Earths". From all the exoplanet surveys conducted to date, this grouping would seem to dominate the statistics.

• How sensitive is Cheops?

  It will be concentrating on bright stars, but even so - its observations will still be challenging.

  When a Jupiter-sized planet passes in front of a Sun-like star, the drop in light as viewed by Cheops will be as little as 1% of the total signal. If an Earth-sized planet does the same thing, the drop-off will be a hundred times smaller again, at 0.01%.

  "The difficulty was in building an optical system that is capable of measuring these minute light changes," recalled Prof Willy Benz, the Cheops consortium principal investigator.

  "To give you an example, when we wanted to test this in the lab we didn't find a single light source in the world that was stable to this precision to allow us to test our telescope - so we had to build one."

• How does it fit with other missions?

  The Americans are currently flying a space telescope called the Transiting Exoplanet Survey Satellite (Tess), a follow-on to the highly successful Kepler observatory. Both are planet-finders and have had input into the candidates soon to be pursued by the 280kg Cheops observatory.

  The Nasa ventures have, if you like, provided the shortlist for the European telescope. Its studies will now whittle the targets down still further to find the most promising subjects for the next generation of planet investigators. These missions will have the ability to analyse the chemistry of exoplanet atmospheres, looking for gases that might hint at the presence of life. The most eagerly awaited is the James Webb Space Telescope (JWST) due for launch in 2021.

  "It's very classic in astronomy that you use a small telescope 'to identify', and then a bigger telescope 'to understand' - and that's exactly the kind of process we plan to do," said 2019 Physics Nobel laureate Prof Didier Queloz.

  "Cheops will now pre-select the very best of the best candidates to apply to extraordinary equipment like very big telescopes on the ground and JWST. This is the chain we will operate."

  Prof Queloz, who co-discovered the very first planet around a Sun-like star, is chair of the Cheops science team.

  The 30cm-aperture telescope was a secondary passenger on the Soyuz launch. The primary payload is an Italian radar satellite.

  One of the early tasks for controllers, once the telescope is safely in orbit, will be to open a protective door over the optics.