This week, in an event hosted by Fundación madri+d, David Barrado and Jorge Lillo, from the CSIC-INTA Astrobiology Center, share with OpenMind one of the most spectacular subjects in astrophysics over the last century: the search for exo-Earths. What are exoplanets and why have they become a priority for sky scientists?
While the early 20th century saw the golden age of fundamental physics, with quantic physics in the limelight, at the beginning of this century stars with exoplanets are shining in their own right, no pun intended.
Since the first exoplanets were discovered in the mid-nineties, this scientific field has made great advances. The existence has been confirmed of over 800 planets that orbit stars far away from our own Solar System. This has allowed us to understand the immense variety of planets, their possible formation scenarios, their sizes and mass. Furthermore in some cases we are already studying their atmospheres, down to their chemical composition. In the majority of cases these are giant planets essentially gaseous or composed of ice. The similar planets in the Solar System are Jupiter and Saturn on one hand and Neptune and Uranus on the other. The next step is the study of rocky planets similar to Mercury, Venus, Earth, and Mars. And what can be called the exoplanetary grail: the habitable area, where it is possible to find water either in gaseous, solid, or–the most exciting–liquid form, which hints at the possibility that life will be found. It is aking to climbing a ladder of knowledge in which each rung provides new data along with new questions.
A large number of gaseous planets are already known, that have a greater mass than Jupiter and are generally very hot. This is logical, as they are the easiest to detect given that, although we cannot see them, indirect signs of their existence clearly appear while observing their stars. As this area of research has grown, technology has also developed, with space telescopes like CoRoT or highly precise instruments on earth such as HARPS, and increasingly small planets have been discovered. This scientific and technological race had a double objective: on one hand to reveal exoplanetary architectures (the different types of planetary systems found) and, on the other hand, the detection of planets similar to our own, both in terms of size and atmospheric conditions and habitability. In short, we wanted to find “exo-Earths”.
With this idea in mind, in 2009 NASA launched the Kepler Space telescope, designed specifically to identify Earth-like planets orbiting around stars like our Sun in orbits making possible the existence of liquid water on their surface. Since, this telescope is pointed at a single small region of the sky exclusively, taking images every 30 minutes of about 150,000 stars. In this way, measuring very precisely the variations in the brightness of each of the stars, it is possible to detect objects that partially hide the stars upon passing in front of them and produce reductions of luminosity, as in Solar eclipses. This is called the transient method. It is exactly what can be observed when Mercury or Venus are projected over the solar disk. In practice this is not noticeable and daylight remains given that Venus and Mercury are very small compared to the sun. But when measured with adequate instruments, a reduction of the brightness of our star can be observed. With exoplanets the procedure is the same and, depending on the magnitude and duration of the reduction, critical characteristics of the planet can be calculated such as its radius, its orbital period, or the distance that separates it from its star.
The precision of the Kepler space telescope is such that it can detect reductions in the brightness of a star of the order of 10 parts per million. In other words, it could detect, at a distance of 30 kilometers, a fly resting on one of the windows of the Empire State building. With such precision, the data from the Kepler telescope reveal the existence of hundreds of potential planets whose size is similar to the Earth. In fact, in December 2011, the confirmation was announced of the planet Kepler-22b, a super-Earth in the habitable zone of its star that appeared to be a giant ball of water. Since then, the Kepler team has been able to confirm various planets of the size of the Earth.
However, the confirmation that this lowering of brightness is truly produced by a planet first requires the determination of its mass, given that different stellar configurations exist that can imitate the signal produced by said planet. And, for the most part, the mass can only be determined by two techniques: the radial velocity method and the astrometric measurements (small variations of the position of the star in the sky’s plane). Both methods measure the swaying of a star due to the gravitational attraction that the planet exercises over it. This explains why the smaller the planet (or the smaller its mass) the more difficult it is to detect and consequently to confirm.
On 20 February 2013, an international group of scientists headed by Dr. Thomas Barclays in which the authors of this article participated announced the validation of the smallest planet known to date. This planet is actually found in a system of three planets orbiting around a star somewhat smaller than our Sun called Kepler-37. The planet furthest from its star (Kepler-37d) is a super-Earth; the next closest (Kepler-37c) is of a similar but somewhat smaller size than our Earth; and the closest (Kepler-37b) is smaller than Mercury, with a size very close to that of our moon. The three planets have orbits too close to their sun to host life on their surfaces (at least life as we know it). The superficial temperatures are so high that water could not exist in liquid form.
The Spanish part of the team has been charged with obtaining very high resolution images of the guest star with the objective of discarding other possible stellar configurations that could imitate the signal produced by the planet. To this end, they have used the technological capacity of the Spanish-German Observatory at Calar Alto (Almería), in particular the 2.2-meter diameter telescope installed at the site. The instrument employed is called AstraLux and employs a technique called lucky-imaging that obtains tens of images per second of the star with the objective of detecting potential weaker objects closer to Kepler-37. This techniques makes it possible to obtain images of a similar quality to that obtained by the Hubble space telescope. The results of this study are of great use for subsequent analysis, using a program called BLENDER, of the probability that the object observed in transit is actually a planet.
The detection of Kepler-37b took the Kepler telescope’s detection capacities to the limit (the brightness decrease in the star is only 20 parts per million). But it also opens the door to the detection of dwarf planets and even moons. As previously indicated, numerous giant planets have been found, some of which are in the habitable zone. However, given their gaseous nature they have been discarded as possible life-hosting planets. The detection of planets of lunar size (such as Kepler-37b) opens a new door, allowing the detection of possible moons that would orbit these gaseous systems (like the satellites Europa, Io, Calisto, and Ganimedes–larger than Mercury–near Jupiter, or Titan in Saturn’s case), which could present the necessary conditions for the existence of liquid water in their surface or interior. This is believed to be the case for some of the moons of the gaseous giants of the Solar System such as, for instance, Europa or Enceladus (the moons of Jupiter and Saturn, respectively) that could have oceans of liquid water under their icy surfaces.
Only 20 years ago it was inconceivable to detect even planetary mass objects and 10 years ago it was difficult to state whether we would become capable of detecting earth-like planets, and we didn’t even consider the detection of planets smaller than those of the Solar System, The question today is what the near future might bring in the exoplanetary research area. Only a year ago the first planet of the size of the Earth was discovered and we are now capable of detecting lunar-size planets. We are getting closer to the final rung of the ladder: a rocky planet with water oceans. After all, the Solar System planets are not the only ones and the discovery of a twin of the Earth in terms of conditions and habitability may not be so remote.
To access the original version of this article published in madri+d, click here.
David Barrado
Astrobiology Center CSIC-INTA
Jorge Lillo
Astrobiology Center. CSIC-INTA
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