Your Guide to CHEOPS

Measuring the diameter of exoplanets

Mission Lead
Launch Date
18 Dec 2019
Earth Orbit
Current Status
Prime Mission

At a glance

  • CHEOPS, the CHaracterising ExOPlanet Satellite (pronounced "KAY-ops"), is a space mission to precisely measure the diameters of known exoplanets
  • Built and led by European Space Agency (ESA)
  • Launched in 2019; 3.5 to 5 years of science operations expected

Why CHEOPS matters

We now understand that there are about as many planets in our galaxy as there are stars. We’ve detected more than 4,000 of them. Is Earth unique, or are there other worlds that could harbor life? Are there other planetary systems like ours out there, or are we unusual? To answer these questions, we must go beyond detection.

CHEOPS will be the first mission designed to follow up exoplanet discoveries, characterizing already-known exoplanets. The first facts we need about exoplanets are their diameters and masses. From diameter and mass we can calculate a planet's density, which tells us whether an exoplanet is rocky like Earth, mostly gas like Jupiter, or in between, like Neptune. CHEOPS will measure the diameters of known exoplanets.

CHEOPS above Earth (Artist's Impression)
CHEOPS above Earth (Artist's Impression) ESA/ATG medialab

How CHEOPS will measure exoplanet sizes

Like its predecessor missions Kepler and TESS, CHEOPS will examine exoplanets using the transit method, watching for dips in the light from other stars as exoplanets pass in front of them. Unlike Kepler and TESS, CHEOPS will study exoplanets that have already been found. 

Exoplanet Brightness Dip During Transit
Exoplanet Brightness Dip During Transit As an exoplanet transits its host star, the star's brightness temporarily dips. Measuring the brightness change helps scientists determine the exoplanet's diameter. ESA

CHEOPS will launch into a polar orbit, positioned roughly along Earth's terminator where day turns to night. The spacecraft has a single science instrument: a specialized camera called a photometer. Starlight reaches the photometer via a 32-centimeter-wide telescope. The telescope will point directly away from the Sun for observations and use a sunshield to stay cool. 

CHEOPS will observe transits of Earth-to-Neptune-sized exoplanets and measure their diameters within 10 percent. That information, combined with mass estimates from other exoplanet-hunting techniques like radial velocity, should tell us what more of these worlds are like. The most common size of exoplanet lies between Earth and Neptune—super-Earths or sub-Neptunes—and we don’t yet know whether that type of planet can host a habitable environment. CHEOPS will help us identify those worlds so that current and future space- and ground-based telescopes can then take a closer look at their atmospheres.

How big is CHEOPS?

CHEOPS is about the size of a golf cart, measuring just 1.5 meters high with a hexagonal footprint 1.6 meters across. It has a mass of 280 kilograms with propellant. 

CHEOPS before Flight
CHEOPS before Flight CHEOPS, the CHaracterising ExOPlanet Satellite, sits in a clean room at Airbus Defence and Space in Madrid, Spain before launch. ESA / S. Corvaja

All Planetary Society resources

External resources