In recent years, researchers focused on the search and study of exoplanets, planets that orbit a star other than the Sun. All eyes are on celestial bodies in the habitable belt, at a distance from the star that allows water to remain liquid on the planet’s surface. Among all the known exoplanets, those potentially suitable for life are only a few dozen. The factors to decide if they are plausible candidates are many, including the internal temperature of the planet. Also, proximity to their star that could make life impossible if emitting enormous quantities of UV rays and X rays.
The dawn of exoplanets research
In 1995 astronomers Didier Queloz and Michel Mayor discovered 51 Pegasi b, the first exoplanet found orbiting a star like our Sun. This confirmed that planets like Earth could exist elsewhere in the universe and started the hunt for them. After confirming the existence of 51 Pegasi b, a team led by Paul Butler re-analysed the observations on radial velocity, having not predicted large planets orbiting so close and fast around their stars. In 1997, they announced two more plausible exoplanets, 70 Virginis and 47 Ursae Majoris. They had orbits of 116 days and 2.5 years respectively, and their solar systems looked similar to ours. Butler’s team discovered at least 70 exoplanets over the next decade. They were joined by dozens of research projects, sending the count of exoplanets now to hundreds of sightings.
Kepler Space Telescope
NASA’s Kepler Space Telescope, launched in 2009, observed about 150,000 stars. It captured small variations in the amount of light from individual stars, caused by the planets passing in front of them, using photometry. Of particular relevance was a study led by Buchhave and published in 2012. Analysing 226 planets, they discovered that almost all of the gaseous giant exoplanets were around stars with high abundance of heavy elements. They found that gas giants tend to form from proto-stellar clouds with a high metal content. This allows the formation of rocky nuclei, which require large quantities of elements such as magnesium, silicon and iron. The nuclei gravitationally accumulate a gas envelope, until they become gas giants.
Kepler’s data confirmed that gas disks around young stars dissipate within a few million years. This means that planets that form in low-metallic environments may not reach sufficiently large core masses before the gas disc dissipates. It may explain why we find very few gas giants around stars with low metal content. Since Earth-like planets don’t require such chemical conditions for their formation, they could be present homogeneously in our Galaxy. Overall, the mission has so far returned more than 3,500 confirmed exoplanets, including 2,400 planetary candidates. Kepler’s four years of data are still revealing new planets, but the failure of some parts of the spacecraft forced NASA to prematurely end the mission in 2013.
CoRoT (Convection Rotation and planetary Transits) was a mission of the French space agency CNES in collaboration with ESA. It was the first space observatory for the search of exoplanets with the transit method, also designed for helioseismology research. The vehicle was equipped with a 27 cm diameter telescope for detecting exoplanets through photometric observation of their transits in front of their stars along the line of sight. Launched in 2006, the orbiting observatory’s operational period was extended until 2013. In November 2012, the on-board electronic instrumentation suffered damage by radiation; the unsuccessful attempts to recover the satellite’s functionality led to the decision to deactivate it.
In April 2018, NASA launched the Transiting Exoplanet Survey Satellite (TESS), a successor to Kepler. With four cameras, it can scan the whole sky at 360 degrees. In just over a year, TESS identified over 1,200 planetary candidates, 29 of which astronomers have already confirmed as such. The mission discovered earth-sized TOI 700 d, part of a triple planetary system around star TOI 700. The planetary system of TOI 700 is relatively close to us; the star is a small red dwarf, smaller and less luminous than the Sun, with a mass about 40% of that of the Sun. TOI 700 d is the outermost planet and the only one in the habitable zone. It has a radius 20% larger than the Earth’s and turns around its star in just 37 days. Its star is a quiet star, almost like our Sun, with minimal variations in brightness.
In 2019, the European Space Agency launched CHEOPS (CHaracterising ExOPlanet Satellite); its purpose is to study and analyse exoplanets that have already been discovered. Unlike TESS, ESA’s satellite will study the elements that make up the celestial bodies, allowing astronomers to determine whether they are rocky or gas giants.
CHEOPS shares research method with its predecessors, observing transits of planets in front of the stellar disk. What the transit method records is a very slight drop in brightness of the transiting star, due to the small portion of the stellar disk prospectively hidden by the planet that is passing in front of it. The mission will target relatively bright stars to obtain measurements of the planetary radii. The planets are selected from two main categories: Earths and super-Earths. Super-Earths are rocky planets with a mass 2-10 Earth masses and a radius 1-1.75 Earth radius.
Instruments will also be able to discover new exoplanets by measuring differences in the observed duration of transits compared to the expected duration. Thanks to its remarkable photometric accuracy, CHEOPS could discover the existence of exomoons or ring systems.
CHEOPS will also be able to obtain the phase curves of ultra-hot Jupiters. That is, the brightness variations recorded during an entire orbit around the star. This type of observation allows to obtain important information on planetary atmospheres. For example, cloud cover, winds and the transport of heat from the illuminated hemisphere of the planet.
At last, part of CHEOPS time will be devoted to observe stars that are orbited by planets of which we know the existence only through the spectroscopic measurements of their radial velocity. It will verify if any of those planets also transit in front of the stellar disk with respect to our terrestrial observation point.
In recent times we have witnessed the discovery of planets similar to Earth. They are potentially able to host life, but no one appears an ideal candidate to suggest that life as we know it can exist there. This doesn’t mean that such an exoplanet (or perhaps more than one) doesn’t exist. That is why ESA has decided to give a boost to the hunt for similar earths with Plato (Planetary Transits and Oscillations of stars), set to launch in 2024. Thanks to its cameras and the 34 telescopes, the mission will monitor nearby stars looking for small variations in brightness produced by the passage of exoplanets. Plato’s analysis will be combined with observations from Earth to estimate the size and density of the newly discovered exoplanets.
ESA will also launch the mission Ariel in 2028 to make a census of the chemical composition of exoplanets’ atmospheres. This will allow to better understand the formation and evolution of planets, and perhaps will confirm the existence of planetary systems similar to ours.