Making detailed measurements of planets orbiting nearby stars is the mission of the Characterizing Exoplanets Satellite, or CHEOPS.1 The European Space Agency (ESA) recently selected CHEOPS for use in its Cosmic Vision Program, a 10-year series of exploratory space missions, starting in 2015. We know there are hundreds of exoplanets—planets that orbit stars other than our sun—and have investigated many of them using indirect techniques. However, astronomers lack detailed information about these planets because they have observed so few of them directly. CHEOPS (see Figure 1) aims to characterize these known planets in great detail. ESA has yet to finalize the satellite's first mission profile, but one possibility is to place the system in the Sun Synchronous or Geostationary Transfer Orbit, using the small and newly developed VEGA launcher.
Figure 1. The Characterizing Exoplanets Satellite (CHEOPS) is a Cassegrain satellite with high photometric accuracy. (Photo courtesy of Christopher Broeg, University of Bern, Switzerland.)
CHEOPS would not necessarily discover new planets, although this could occur as a secondary outcome of its mission. Most ‘alien’ planets are discovered through techniques such as radial velocity and transit detection. Measuring the radial velocity of the planet's host star by means of accurate spectroscopy enabled the first firm detection of exoplanets, such as that reported in 1995 by Mayor and Queloz.2 Today, transit photometry provides the largest sample of candidate exoplanets, obtained mostly from the NASA Kepler mission. Direct imaging, although increasingly affordable, can only provide accurate information when combined with these techniques.
Radial velocity detection involves measuring variations in the velocity of a star as it moves in an orbit determined, in part, by the gravitational pull of its orbiting planet. For the method to be accurate, the detected planet must be relatively heavy and close to a bright star. Transit detection—which measures the radius of a planet according to the decrease in visual brightness from a star caused by the orbiting planet ‘transiting’ across its disc—requires the detected planet to have a relatively large diameter and the number of stars sampled to be large, which precludes the use of telescopes with smaller fields of view.
CHEOPS can precisely measure the transit of bright stars that we already know to have an orbiting planet. The detailed characterization of these stars and planets, using existing and developing spectroscopic methods, will allow us to determine with high precision the physical size of a planet or the presence of an atmosphere. This knowledge, in turn, should ultimately help us to classify planets as rocky (like the outer planets of our solar system, including Earth) or gaseous (like Jupiter).
The specific scientific goals of CHEOPS include the following: to determine the mass-radius relation in a planetary mass range for which only a handful of data exists; to probe the atmosphere of known so-called Hot Jupiters (a special class of extrasolar planets); to provide unique targets for future ground- and space-based facilities with spectroscopic capabilities; to offer up to 10% of open time to the community to be allocated through competitive scientific review; to identify planets with significant atmospheres; and to elucidate the migration paths of planets as a key to the evolution of planetary systems.
The CHEOPS system uses relatively small optics of 0.3m diameter, with a focal plane characterized by a small field of view (0.25 square degrees). The light beam is shaped by an axicon-type lens to create a disc, tens of arcseconds in diameter, that produces extremely accurate photometry by averaging visible light over a large number of pixels in a CCD. A uniformly shaped beam achieves accuracy of 100 parts per million in a single minute's exposure of a 9 magnitude star. The satellite maximizes transit time, using pointing accuracy (toward the exoplanet's host star) and mechanisms to maintain the system at an attitude that reduces variations in temperature, to obtain high precision photometry.
The overall objective of CHEOPS is to enhance our understanding of exoplanets that we have already identified. Data it obtains may help exoplanetology move into a precision phase after two decades of discovery. In future, however, we could use the system to identify new planets, or even moons.
Astronomical Observatory of Padova (INAF)
Roberto Ragazzoni is an astronomer working in optical instrumentation. He specializes in adaptive optics, and introduced the pyramid wavefront sensor and layer-oriented approach. He is co-author of more than 100 SPIE papers and has been a committee member for various SPIE conferences.
2. M. Mayor, D. Queloz, A Jupiter-mass companion to a solar-type star, Nature 378, p. 355-359, 1995.