A space-based observatory to hunt for habitable planets around other stars is the leading candidate to win the financial backing of the European Space Agency later this month.

Artist's concept of PLATO. Photo credit: ESA
 
The PLATO mission is tentatively slated to launch between 2022 and 2024 to a position a million miles away from Earth, where it will scan the sky for at least six years with 34 telescopes arrayed together to peer at a vast number of stars searching for the signatures of orbiting planets.

PLATO won the backing of an ESA science panel in January. The news was first reported by the BBC and confirmed by two sources familiar with the decision.

The mission will build on ongoing work by scientists using data from NASA's Kepler observatory, and researchers say PLATO will supply ground-based telescopes and the James Webb Space Telescope with planetary targets for follow-up studies.

PLATO will assemble a catalog of planets, measuring the size, mass, density and age of alien worlds about the size of Earth located at just the right distance from their host stars with hospitable temperatures that could support liquid water and life.

"We want to completely characterize low-mass planets out to the habitable zones, learn about their internal composition, the density, and the age of the system," said Stephane Udry, an astronomer at the University of Geneva, in a presentation of the mission in November.

Udry said PLATO will provide a "huge number" of planets for observations by future missions, which could reveal the nature of their atmospheres and chemical make-up.

Unlike Kepler, which imaged a narrow swath of the sky in the constellations Lyra and Cygnus, PLATO will scan two wide fields, staring at each for up to three years and collecting and analyzing light from a million stars.

"The idea is to observe two big fields for a very long time and use a step-and-stare phase to cover nearly half the sky," Udry said.

PLATO stands for PLAnetary Transits and Oscillations of stars.

ESA's space science advisory committee recommended pursuing the PLATO mission in a competition between five proposed projects:

• The Exoplanet Characterization Observatory, or EChO, is designed to measure the chemical composition and structure of hundreds of planets, ranging in size from gas giants as big as Jupiter to so-called "super-Earths" slightly larger than our own planet.
  
  
• The Large Observatory for X-ray Timing, or LOFT, was proposed to obtain high-time-resolution X-ray observations around black holes, the centers of galaxies and collapsed stars to study how matter behaves at high densities and and under strong gravitational forces.
  
  
• The Space-Time Explorer and Quantum Equivalence Principle Space Test, or STE-QUEST, mission would probe the fundamental physics of the universe, employing a satellite in Earth orbit to test a key tenet of Albert Einstein's Theory of General Relativity by measuring the effect of gravity on time and matter.
  
  
• The MarcoPolo-R mission is a mission to collect and return a pristine sample from a near-Earth asteroid a bit larger than a typical a city block. The probe is designed with a sampling mechanism and a parachute-less cushioned re-entry capsule to deliver the asteroid material to Earth.

The advisory panel's endorsement of PLATO now goes to ESA's science program committee to take up the decision to select the mission for implementation.

PLATO would become the third medium-class mission in ESA's "Cosmic Vision" program, which aims to develop strategic space science probes for solar system exploration and research in astronomy and astrophysics.

The Cosmic Vision missions include large- and medium-class probes. The first large mission chosen by ESA in 2012 was the Jupiter Icy Moons Explorer, or JUICE, a spacecraft to conduct flybys of Jupiter's moons Ganymede, Callisto and Europa. JUICE is set for launch in June 2022.

In December, ESA announced the second and third large-class Cosmic Vision missions will be an X-ray telescope and a long-proposed observatory to confirm the existence of gravitational waves predicted by Albert Einstein's theory of general relativity.

The two medium-class projects selected by ESA so far are Solar Orbiter and Euclid set for launch in 2017 and 2019. Solar Orbiter will obtain the highest-resolution images ever of the sun, and Euclid will map the influence of dark energy on the evolution of the universe.

ESA passed over PLATO in its previous medium-class mission selections, but the agency put the mission back in the mix for the third round.

The candidates for the third medium-class mission opportunity, including PLATO, have a European Space Agency cost cap of 600 million euros, or about $810 million. ESA member states or international partners like NASA could add contributions beyond the cost limit.