PLATO, looking at the stars in search of Exo-Earths

In February 2014 ESA approved a new medium-class mission, PLATO; its main remit is to look for exo-Earths, i.e., Earth-sized exoplanets orbiting within a star’s habitable zone. It could be said, therefore, that PLATO is going to map the heavens looking for a second home for us. Put like that, it certainly sounds thrilling.

A little (only a little) history


Giordano Bruno (Wikimedia Commons)

Most people nowadays are familiar with extrasolar planets or exoplanets, i.e., planets outside the solar system either orbiting other stars or subject to no gravitational influence, like the rogue planets. But just imagine the shock it must have been back in the sixteenth century when the Dominican Friar Giordano Bruno first put forward the theory that the Sun was just another star, and just as our Sun had planets orbiting around it so could any other star. These planets, he argued, could be inhabited by animals and even intelligent life. These declarations, in a world still struggling to come to terms with Copernicus’s revolutionary heliocentric theories, were completely radical, and were one of the reasons (though not the main one, it must be said) that condemned him to be burned at the stake.

The next mention of exoplanets came in the eighteenth century, courtesy of the discoverer of the universal law of gravitation, Sir Isaac Newton. Newton, just like Bruno, drew a comparison between the stars and the Sun with its planetary system, meaning that other stars might well have planetary systems similar to our Sun’s. This could be considered to be the watershed moment after which exoplanets came to be seen as an established possibility within the scientific community. There still remained, however, the tricky task of detecting them.

The beginnings and the first exoplanets

There is no doubt that being the first to discover an exoplanet would be a real feather in anyone’s cap, guaranteeing them a mention in all history books of science in general and astronomy in particular. Claims of exoplanet detection were therefore already being made as far back as the nineteenth century, mainly on the grounds of orbital anomalies. This is a method of proven efficiency, having served to detect the planet Neptune at the start of that century. These claims were unfounded, however, and all those made subsequently, up to the nineties of the twentieth century.

It was the Polish astronomer Aleksander Wolszczan who was the first to detect an extrasolar planet, found orbiting a pulsar (a rotating neutron star that emits a regular beam of electromagnetic radiation). This discovery was made in 1992; shortly afterwards, in October 1995, the Swiss astronomers Michel Mayor and Didier Queloz discovered the first exoplanets orbiting a star of the main sequence, followed a few months later by two new planets discovered by a team from California University.


Artistic Recreation of Exoplanets (ESA – C. Carreau)

The race had started. From these hesitant beginnings, with announcements of discoveries that then had to be detracted, a total of nearly 2000 exoplanets have been confirmed to date, duly catalogued in The Extrasolar Planets Encyclopaedia. As a final footnote to this section it is worth pointing out too that some exoplanets originally misidentified as other astronomical bodies back in the eighties have now been phased into the exoplanet catalogues.

Detection Methods and Types of Exoplanets

How are exoplanets detected? The first idea that might spring to mind is to look through a very big and powerful telescope. Unfortunately, the planets are very dim and are also hard by a star that shines billions of times more strongly, so they cannot be picked up by our instruments. Some images of exoplanets have been obtained, however, usually in the infrared spectrum and involving very big, still hot planets a long way from their stars. As might be imagined, this does not happen often.

The most usual detection methods are indirect ones, i.e., detecting the planet from its effects elsewhere, normally on its star. The most widely used of these indirect detection methods is the transit method. This involves analyzing the variations in a star’s brightness, possibly caused by an orbiting planet in transit across it. Another method is called radial velocity, whereby a planet with sufficient mass, and not too far from its star, can vary the star’s orbit, producing a Doppler effect detectable from Earth. Other methods are astrometry, transit timing variation, gravitational microlensing, etc.

All these methods have given rise to a surprising variety of exoplanets. Apart from the known planets of our solar system, which could be broken down basically into terrestrial or rocky planets, such as the inner planets of the solar system, and gassy giants , like the outer planets, other systems might include Hot Jupiters and Hot Neptunes (planets of the size of Jupiter or Neptune very close to their star), Super-Earths, chthonian planets, carbon planets (also going under the suggestive name of diamond planet), Mini Neptunes , ocean planets, and many other different types. As Mr. Spock (played by the recently deceased Leonard Nimoy) might say: “Infinite diversity in infinite combinations”.


Comparison of planet sizes with different compositions (Marc Kuchner/NASA, edited por Henrykus/Wikipedia)

Comparison of planet sizes with different compositions (Marc Kuchner/NASA, edited por Henrykus/Wikipedia)

What have space missions got to do with all of this?

A very good question. Very good indeed. Several missions have helped to boost the number of exoplanets discovered. The first mission that set out with this purpose was COROT (aconym in French of COnvection ROtation et Transits planétaires: COnvection, ROtation and Planetary Transits). As the name itself suggests, COROT used the transit method to detect new planets (32 confirmed, several still pending). This was a joint mission of the French Space Agency (CNES in French initials) and the European Space Agency (ESA), following a polar orbit.

Probably the most famous mission of all is KEPLER of the North American Space Agency (NASA), which also used the transit method for detecting exoplanets. Kepler followed an Earth-like orbit around the sun to avoid the perturbations of an actual Earth orbit. Focusing permanently on the same heavenly area, it has by now confirmed the existence of over one thousand planets, some of them probably rocky and lying in the parent star’s habitable zone, plus several thousands of as yet unconfirmed planets. Problems with its reaction wheels (used for precise pointing of the satellite) brought its main mission to an end; the subsequent switch to its secondary mission means that few more exoplanets are now likely to be detected.

Brief interlude. What is a star’s habitable zone?

In a nutshell, a star’s habitable zone is the area where life as we know it can exist. The more complete definition given on the Spanish Wikipedia page (translated into English) runs as follows: “the region around a star that, containing a rocky planet (or satellite) with a mass ranging from 0.6 to 10 Earth masses and an atmospheric pressure higher than 6.1 mb, around the triple point for water, the luminosity and incident radiation flow allow the presence of water in a liquid state on its surface”, but I think the first definition will do us fine.

It is a very controversial definition and the minimum and maximum distances of this region are hotly debated. They also vary from star to star; lying within this zone is also a necessary but not sufficient condition, since the planetary conditions themselves may not be compatible with life. Mars, for example, lies within the Sun’s habitable zone (according to some estimations), but has not generated life (to the best of our knowledge).

Example of an extrasolar planet (this particular planet has the poetic name of 55 Cancri f) lying with its star’s habitable zone, marked here in green (NASA/JPL-Caltech)

Example of an extrasolar planet (this particular planet has the poetic name of 55 Cancri f) lying with its star’s habitable zone, marked here in green (NASA/JPL-Caltech)

Next step in Europe, the PLATO mission

PLATO, standing for PLAnetary Transits and Oscillation of stars, is a medium-class ESA mission using the transit method to detect exoplanets, with a greater definition precision than in earlier missions.

The mission has had a checkered history. In 2011 it was presented to ESA but was turned down in favor of the EUCLID mission (which will analyze dark energy) and the Solar Orbiter mission (to study the heliosphere). Presented anew in February 2014 it was then given the go ahead.

PLATO will be launched on a Soyuz rocket from Europe’s Spaceport in Kourou (French Guiana) for an initial six-year mission. It will operate from Lagrangian point L2, a virtual point in space 1.5 million km beyond Earth as seen from the Sun. From there it will observe up to a million relatively nearby stars in an area covering half the sky. To do so it will be fitted with 34 separate small telescopes and individual cameras.

Location of Lagrangian point L2 of the Sun-Earth system (ESA)

Location of Lagrangian point L2 of the Sun-Earth system (ESA)

When coupled with Earth-based radial velocity observations, PLATO’s readings will allow a planet’s mass and radius to be calculated, and therefore its density, giving an indication of its composition. The mission will identify and study thousands of exoplanetary systems, with an emphasis on discovering and characterizing Earth-sized planets and super-Earths in the habitable zone of their parent star.

Concept of the PLATO mission of the previous phases (Thales Alenia Space)

Concept of the PLATO mission of the previous phases (Thales Alenia Space)

The mission will also investigate seismic activity in the stars, determining such characteristics as mass, radius and age. As well as boosting our knowledge of the stars, this research will also allow us to determine the mission-detected exoplanets’ size with much greater precision (greater, for example, than the KEPLER mission or the mission that could be considered to be its substitute in NASA, the TESS mission).

GMV and the search for exoplanets

GMV has now collaborated with several of these thrilling exoplanet-search missions, playing some sort of part in all European missions to date. In COROT, the first of these missions, GMV was responsible for the design and development of mission control, in charge of preparing and planning scientific activities and also the reception and processing of the mission’s science telemetry.

GMV was likewise involved in the Gaia mission. This mission’s prime remit was astrometry, though it also allowed some extrasolar planets to be discovered. GMV was involved in the project from the word go, assessing the validity of data reduction systems. In later phases it worked on the development of the science operations center and also the initial data processing operations.

Artistic Recreation of Gaia (ESA)

Artistic Recreation of Gaia (ESA)

Another mission on which GMV is collaborating is CHEOPS (CHaracterising ExOPlanets Satellite). Over a three-and-a-half year period this satellite, due for launch in 2017, will search for planets orbiting nearby bright stars, using the transit method. Its particular aim will be the precise measurement of the radii of exoplanets whose mass has already been estimated, giving a reliable idea of the planet’s composition. GMV was involved in the previous phases, providing mission analysis support; it is now working on the development of the operational simulator and the mission control system.

The PLATO mission, for its part, is now in the preliminary definition phase, in which several company consortia are defining mission elements on a competitive basis. GMV, forming part of the industrial team under the responsibility of OHB, is in charge of requirement analysis, critical review of the mission analysis, fast camera characterization and operations support.

Searches for far-off Earths

The search for a twin earth, like the one now being carried out by PLATO, is a spine-tingling adventure. Finding a twin planet means that this planet could well harbor life and might even have established civilizations. There is a controversial statistical formula known as the Drake Equation for assessing the number of intelligent active, communicative extraterrestrial civilizations. It pools several factors such as the rate of star formation, the fractions of stars with planets, the fraction of planets in the habitable zone, etc. These terms are as yet still quite fuzzy and uncertain; even so, current estimates range from a low of 2 to a high of 280,000,000 civilizations in the Milky Way (though these findings, as already pointed out, are fiercely debated). Finding a twin earth, therefore, could be the first step towards finding the Vulcans, the Minbari or something completely beyond our powers of imagination.

Possible exo-Earths, i.e., small planets orbiting the habitable zones of their parent stars, discovered by the Kepler mission (NASA Ames/W. Stenzel)

Possible exo-Earths, i.e., small planets orbiting the habitable zones of their parent stars, discovered by the Kepler mission (NASA Ames/W. Stenzel)

But leaving aside the possibility of intelligent civilizations or life, the search for these so-called Goldilocks planets also represents a quest to find a future home for humanity. True it is that today’s space travel would not allow us to reach them within a reasonable time, or even within an unreasonable period of time, come to think of it, but we can at least make a start on mapping and selection. Who knows what future technological progress might bring? As one of the pioneers of rocket science and father of Russian cosmonautics, Konstantin Tsiolkovsky, once said, “The Earth is the cradle of humanity, but mankind cannot stay in the cradle forever”.

Author: Javier Atapuerca
Head of Mission Analysis and Studies Section (GMV)

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Las opiniones vertidas por el autor son enteramente suyas y no siempre representan la opinión de GMV
The author’s views are entirely his own and may not reflect the views of GMV

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