By

  Professor (Dr) Surjit Singh Bhatti

  Head, Physics and Dean, Applied Sciences (Retired)

 Guru Nanak Dev University, Amritsar (India)

Since the beginning of the 20^th century, Scientists have been trying to find planets outside our solar system, called exoplanets. However,  the first success came in 1995 with the discovery of an exoplanet, called 51 Pegasi b, orbiting around a solar-type main-sequence star in our home galaxy, the Milky Way. For this, the 2019 Nobel Prize in Physics was belatedly awarded to Michel Mayor of Geneva, Switzerland, and Didier Queloz of  Cambridge, UK. This exoplanet is about 50 light years away from the Earth and rotates once around its Sun, 51 Pegasi,  in 102 hours.

The European Space Agency (ESA) is actively engaged in the study of new exoplanets. In December 2019, it launched a powerful new telescope called CHEOPS (which stands for CHaracterizing ExOPlanet Satellite) aboard the Soyuz-Fregat rocket from French Guiana. It has been put in a pole-to-pole orbit about 500 miles above the Earth. It will encircle the poles along the line of division between day and night on the Earth, with the cameras pointing away from the Sun.

Other satellites, carrying powerful telescopes, like Kepler (launched in 2009 and expected to work till 2030), TESS (Transiting Exoplanet Survey Satellite, launched in April 2018) and CoRoT (2006-2013) have been already looking for the exoplanets. CoRoT satellite, which  preceded Kepler, used the transit method to find numerous planets during its functional period. Using data from Hubble Space Telescope, water vapor has been found on an exoplanet in the habitable zone.  Hubble Telescope was put in the (West to East) orbit around the Earth at a height of 350 miles and had succeeded in discovering the existence of several exoplanets around other stars in the universe.  

PLATO  ( PLAnetary Transits and Oscillations of stars) is another Space Telescope under development by ESA and is expected to be launched in 2026. Its objective is to find habitable earth-like planets around Sun-like stars where liquid water is available. It will operate in the visible Spectral range of 500 to 1000 nm.  

CHEOPS  Telescope is designed to study details of exoplanets discovered  by ground- based astronomers like Mayor and Queloz and the Satellites in space. The aim is to find how these worlds were formed. It will spend about four years looking beyond our solar system for exoplanetary transits. These transits are  dips in the light coming from the host star, when a planet crosses in front of it. Size of  the planet determines how much light it blocks and the extent to which the star darkens. This enables Scientists to deduce the sizes of the exoplanets.  Bottom of Form

Mass of an exoplanet is obtained by ground-based telescopes that record the way the star is perturbed by a planet’s gravity. Size and mass allow calculation of each exoplanet’s density. Tracking the star-light before and after a transit helps to find its composition. There are more than 4000 confirmed exoplanets discovered by the Kepler and TESS and a number of more planets are being discovered by ground-based telescopes. About 97% of all the confirmed exoplanets have been discovered by indirect techniques of detection, mainly by radial velocity measurements and transit monitoring techniques. Some exoplanets are believed to be potentially habitable, as liquid water has been detected.  In some cases, multi-planetary systems have also been found to exist around some stars.  

Radial velocity measurements technique can detect planets around slow-moving and low-mass stars, such as red dwarfs (stars smaller than our Sun) which are more affected by the gravitational pull of the exoplanets. There are about 70%  of such stars in the galaxies. This method can also distinguish the planet’s own spectral lines from those of the host star. James Webb Space Telescope (JWST) proposed to be launched in March 2021, can help in this mission using its infrared (IR) instruments to detect the  exoplanets, to be confirmed using the TESS. There is at least one Earth-size planet around every star, implying a trillion planets in our galaxy alone.

Discovery of Water on an Exoplanet ( K2-18b) in 2019  

Water has been detected as vapor in the atmosphere of an exoplanet called  K2-18b,  about 110 light years away. This exoplanet is twice as large in size as our Earth and is about eight times as massive. K2-18b is orbiting a red dwarf star, completing its one full revolution in 33 days. It was first discovered in 2015 by the Kepler Space Telescope. Discovery of water has been made in 2019  by the group of Angelos Tsiaras at the University College of London. They discovered not only the molecules of water but also hydrogen and helium on this exoplanet. Calculations show that its temperature is in the range of about  -72 Celsius to 47 degrees  Celsius. It is believed that its atmosphere could also contain nitrogen and methane. There could potentially be many more exoplanets similar to this one. Its large size is a factor in favor of habitability as water can stay for long on its large surface.  It can retain its atmosphere for a longer time because of its bigger gravity.   The James Webb Space Telescope (JWST) will be able to examine the atmospheres of exoplanets.

 ARIEL (Atmospheric Remote-sensing Infrared Exoplanet Large-survey) is a mission which ESA will launch in 2028 to study the atmospheres of about 1000 exoplanets in detail. This and JWST will give us a much better understanding of K2-12b and other exoplanets like it and the possibility of life on these.