Where do we stand with the search for extrasolar planets suitable for harboring life? Well, all we can say right now is that the technology behind this field of research is still quite “young”. Every day there are new discoveries, but many of them need confirmation. However, we have already managed to put together a good number of planets with characteristics very similar to those of our Earth. at least apparently, and except errors always possible. So much so that we have already found a way to draw up rankings based on factors capable of indicating their habitability, or at least their similarity with the Earth. What we propose is the most updated! While we have barely scratched the surface, we are at a pivotal time in our search for life elsewhere in the Universe. This is largely due to the way modern telescopes have allowed us to discover thousands of extrasolar planets (or just exoplanets). As the number of confirmed exoplanets has grown, the focus has been slowly shifting from discovery to characterization. In other words, we’ve found many distant worlds, now we’re attempting to determine which of them might be able to actually support life. To date, astronomers have confirmed the existence of 4,422 planets beyond our Solar System. Of these, the vast majority have been a combination of Neptune-like gas giants (1,496), Jupiter-like gas giants (1,400), Super-Earths (1,356). Only 165 have been rocky planets that are similar in size to Earth (or “Earthlike”). Of all the planets we have discovered, only 59 have been identified as being capable of supporting life – what astronomers refer to as “potentially habitable.” Most of these (37) fell into the range of Super-Earths to “mini-Neptunes”, 21 were similar to Earth, and 1 was about the same size as Mars. Not bad considering that all of these discoveries have taken place in just over thirty years. But in truth, most were discovered after 2009 when the Kepler Space Telescope was launched. Since then, several missions have built upon this impressive legacy, and more are still to come… In the coming years, we stand to many more planets and learn a great deal more about the ones we already know about. But first, a few things need to be clarified, not the least of which is terminology. For example, what does “Earthlike” mean? Simply put, Earthlike planets are those that are believed to be similar in structure and composition to Earth. Earth is composed primarily of silicate minerals and metals that are differentiated between a silicate crust and mantle and a metallic core. The technical term for this type of planet is “terrestrial”, though astronomers often use the term “rocky” to differentiate them from gas giants (which are primarily composed of hydrogen and helium with some heavier elements concentrated in the core). Beyond structure and composition, “Earthlike” is also meant to imply that a planet has conditions similar to that of Earth. This would include the presence of a thick atmosphere and liquid water on its surface. And… What about “potentially habitable”? This term has also seen a lot of use in recent years whenever the subject of exoplanets comes up. What it refers to are those exoplanets that have been found orbiting within their star’s circumstellar Habitable Zone, which is sometimes referred to as the “Goldilocks Zone.” But how do we determine habitability? Concepts like the “habitable zone” – the orbital distance from a star allowing a planet, with a suitable atmosphere, to retain liquid water on its surface – are helping astronomers sort through the many possibilities to find likelier candidates for life-bearing worlds. Still, planets with life could be far outside this zone if there were, for example, an ice-covered planet with a deep ocean supporting aquatic organisms. A good way to estimate how habitable a planet is the Earth Similarity Index (ESI). Proposed in 2011, this index gives alien worlds a number from 0 (no similarity) to 1 (very similar to Earth) based on a few characteristics. These include the exoplanet’s bulk density, mean radius, surface temperature, and escape velocity (this is essentially the speed at which you must travel to break free of a planet’s orbit). Planets that score above 0.8 are considered to be Earthlike. To give you an idea of how the scale works, Venus scores a 0.44 on the ESI, whereas Mars comes in at 0.64-meaning neither are particularly habitable or Earthlike. Of course, we don’t always have all of the information about every exoplanet, as we glean all of these measurements from analyzing, for example, the light exoplanets reflect. So some of the numbers are our best guestimates. Other factors must also be taken into consideration: Does the planet have a solid surface, an atmosphere, and a magnetosphere to protect it from the harshness of space? Is there some sort of energy source (like plate tectonics)? What chemical compounds make up the planet’s atmosphere? Does the planet maintain a stable orbit around its star that keeps it in the Goldilocks zone permanently? Is its host star stable in its luminosity fluctuations? Does it have an extreme tilt on its orbital axis? For many planets that have been detected, much of this information is unknown. In the coming years, next-generation telescopes are going to be aimed at confirmed exoplanets that have deemed worthy of follow-up observations. Using the Earth Similarity Index as a metric, the following exoplanets would seem like a good place to start. Here they are, the top five exoplanets to watch for in the next few years: “Hey, guys, just a moment before we continue… BE sure to join the Insane Curiosity Channel… Click on the bell, you will help us to make products of ever-higher quality!” 5 fifth place, with an Earth Similarity Index of 0.91: Luyten b! Luyten b is a rocky exoplanet orbiting within the habitable zone of the nearby red dwarf Luyten’s Star, in the constellation Canis Minor (just to the right of the bright star Procyon). With a visual magnitude of 9.9, it’s too faint to be viewed with the unaided eye. Luyten b is one of the most Earthlike planets ever found and is the fifth-closest potentially habitable exoplanet known, at a distance of 12.2 light-years. Only Proxima Centauri b, Barnard’s Star b, Ross 128 b, and Gliese 1061 d are closer. Discovered in June 2017, Luyten b is a Super-Earth of around 2.89 times the mass of Earth. Receives only 6% more starlight than Earth, which makes him one of the best candidates for habitability. The planet is likely terrestrial, with a diameter of about 18000 km and an equilibrium temperature of 14 °C. For comparison, Earth has an equilibrium temperature of 18 °C. Luyten b orbits quite close to its host star. One full revolution around Luyten’s Star takes about 18.6 days at an average distance of 0.091 AU, much closer in than Mercury, which has a year of 88 days and an orbital radius of 0.387 AU. 4 fourth place, with an Earth Similarity Index of 0.94: Gliese 581 g! Gliese 581g, discovered in 2010, is an extrasolar planet that orbit the star Gliese 581, approximately 20.4 light years from Earth in the constellation Libra. It is notable as it is thought to be in the middle of the habitable zone of its star. One full revolution around Gliese 581 takes about 32 days at an average distance of 0.13 AU. The planet is a Super-Earth of around 2.5 times the mass of Earth, and is thought to be tidally locked to its star. The actual habitability of the planet depends on the composition of its surface and the atmosphere. It is thought to have temperatures around -37 to -11 °C. By comparison, Earth has an average surface temperature of 15 °C – while Mars has an average surface temperature of about -63 °C. 3 third place, with an Earth Similarity Index of 0.95: TRAPPIST-1e ! TRAPPIST-1e is an almost Earth-sized terrestrial extrasolar planet orbiting around the ultra-cold dwarf star TRAPPIST-1, located approximately 39 light-years away from Earth in the constellation Aquarius. The exoplanet, discovered in 2016, is within the star’s habitable zone, and it’s one of seven exoplanets to be discovered orbiting the star. More detailed studies released in 2018 determined that the planet is one of the most Earthlike worlds found. It is extremely similar to Earth physically, with 91% the radius, 77% the mass, 102.4% the density, and 93% the surface gravity of Earth. The planet orbits its host star quite closely. One full revolution around TRAPPIST-1 only takes 6 Earth days to complete at a distance of 0.03 AU. TRAPPIST-1e is confirmed to be a terrestrial planet with a solid, rocky surface. It is cool enough for liquid water to pool on the surface. The planet receives a stellar flux 0.6 times that of Earth, about a third lower than that of Earth but significantly more than that of Mars. Its equilibrium temperature ranges from -48 °C to -27 °C, depending on how much light the planet reflects into space. Both of these are between those of Earth and Mars as well. TRAPPIST-1e is confirmed to have a compact, hydrogen-free atmosphere like those of our Solar System’s rocky planets, further raising the chances of habitability. Hydrogen is a powerful greenhouse gas, so if there was enough to be easily detected, it would mean that the surface of TRAPPIST-1e would be completely inhospitable. Since such an atmosphere is not present, it raises the chances for the planet to have a more Earthlike atmosphere instead. 2 second place, with an Earth Similarity Index of 0.95: Teegarden b ! Teegarden b is an exoplanet found orbiting within the habitable zone of Teegarden’s Star, an M-type red dwarf in the constellation Aries, about 12 light-years from the Solar System. Teegarden’s Star b, discovered In July 2019, is the innermost known planet orbiting Teegarden’s Star, with an orbital period of just 4.9 days at an average distance of 0.025 AU. Teegarden b orbits within the habitable zone of its host star. This means that its atmospheric composition could allow for stable liquid water on its surface, and It is thought to have surface temperatures around -27°C The planet’s minimum mass and diameter are identical to those of Earth. Because of this, Teegarden b is likely to be rocky, with an ocean of water on its surface. 1 first place, with an Earth Similarity Index of 0.98: KOI-4878.01 ! Discovered in 2015, KOI-4878.01 is an extrasolar planet that orbits the F-type main-sequence star KOI-4878. It is located about 1075 light years from Earth in the constellation Draco. The characteristics of the planet are very similar to that of Earth, and if it is confirmed, would have an Earth Similarity Index rating of 98%, which makes it the most Earthlike planets found. The orbital period of the planet is around 449 Earth days at a distance of 1.12 AU. It is very likely located within the habitable zone of its star. Its mass and diameter are identical to those of Earth, as well as its surface temperature of 18°C. The only flaw, its great distance! Mind you… this is a classification based on fragmentary data, some of which are still quite uncertain: it could change with each new discovery. After all, we are still in the first steps in this difficult remote exploration of planets lost in the brightness of their parent star. In the near future, more and more powerful telescopes will be deployed to search for and confirm more and more life-sustaining planets. First in line is the James Webb Space Telescope, targeted for launch in October 2021. The Webb telescope will be a cosmic multi-tasker, looking deep into the universe – and deep into its past – to discover clues to its origin and early formation. The Webb telescope also will capture starlight shining through the atmospheres of exoplanets, which provides a kind of profile of the gases present. This new era of characterizing exoplanets will continue with the launch of the Nancy Grace Roman Telescope in the mid-2020s. An intricate instrument onboard called a coronagraph will help blot out the glare of parent stars to reveal orbiting planets. That will mean direct images of large, gaseous planets. Those targets are unlikely to be habitable, but demonstrating this technology will open the door to future such instruments with greater resolving power. A future telescope might even find a small, rocky world with an atmosphere of oxygen, methane, and carbon dioxide – in other words, an atmosphere that reminds us of home.