The universe , and the Milky Way in particular, is full of planets . In just three decades we have discovered more than 5,000 different planets orbiting nearby stars, with most of these discoveries in the second half of that period. Some are several times the size of Jupiter, and others are smaller than Earth . Some orbit so close to their star that they have temperatures of more than 1,000 degrees, while others orbit several times the distance . between the Sun and Pluto . In addition to their sizes, there is also great variability in their compositions and densities . It is because of this incredible variety that it is natural to wonder which of these planets are most similar to Earth. This question is not only interesting for a matter of vanity but because, since the only planet with life that we know of is ours, there is the possibility that planets like ours are the most suitable for life . But what exactly is Earth like? What properties make it fit for life?
One of the most fundamental characteristics will be determined by the mass and composition of a planet , which, depending on the specific combination, will give it a different final size. That is, a planet of the mass of the Earth but composed mainly of gases or liquids will have a different size. This will not only be important to categorize the planet, but also because we consider that, for example, it would be more complicated for life to arise on a gaseous planet or on a planet without the presence of compounds in a liquid state , completely desert. This is because biochemistry needs a certain support material on which to act . Molecules need to be able to find each other fortuitously so that the necessary reactions and conditions occur so that, with luck and time, life arises.
Being the rocky planet, the presence or not of liquids will depend on the temperature . This temperature will depend on the distance to the star, but also on the type of star it orbits. Around a star like the Sun, liquid water can survive on a planet’s surface at distances of between 100 and 200 million kilometers , in what is known as the habitable zone. Specifically, the Earth is located almost 150 million kilometers from the Sun. But orbiting around smaller stars, these distances are reduced and we can find planets with liquid water at distances of a few million kilometers from their star, as occurs in the TRAPPIST system. -1 . In addition to the distance and type of star, the composition and density of the atmosphere also matters. Venus, Mars and Earth have very different atmospheres. Those of Mars and Venus have a similar composition, since they are composed almost entirely of carbon dioxide, but very different densities, the atmosphere of Venus being almost 10,000 times denser than that of Mars. Therefore, if we were to place the three planets in an orbit like the Earth’s, Venus would still be too hot and Mars would still be too cold . We would need to reverse their positions, placing Mars where Venus currently is and Venus where Mars currently is for their temperatures to be more similar to Earth’s.
The type of star is not only important for the amount of light it emits, but also for the type of light and the time during which it emits it. The most massive stars, 5 or 10 times the mass of the Sun, could host habitable planets at very great distances, but their lives are so short, just a few tens or hundreds of millions of years, that it is unlikely that life will develop . complex on any of these planets. The smallest stars live for billions of years, giving plenty of time for any life to arise, but some can be unstable, with powerful radiation emissions that can endanger any start of life.
Let us therefore see which planets will be most similar to Earth in each aspect, assuming that all of them are in the habitable zone of their star.
It is difficult to accurately measure the size of a planet , especially for those that are relatively small and not several times the diameter of Jupiter. That is why it is possible that these exoplanets leave this category in the future. Still, we know of several exoplanets with sizes and masses very similar to ours. Two of them orbit the star Teegarden with radii, in principle, barely 2 and 4% larger than that of the Earth and with a slightly higher mass. Also around TRAPPIST-1 a planet with a radius 5% larger than the Earth’s and a slightly lower mass is known. These planets therefore have a density very similar to that of our planet , which guarantees that they will be rocky. However, we believe that TRAPPIST-1 f and Teegarden c would be considerably cooler than Earth. Teegarden b itself could have a temperature very similar to ours, depending on the composition of its atmosphere.
Although we cannot yet measure the temperature on the surface of these planets, we can calculate the equilibrium temperature at the specific distance from their star at which they are located. This is the temperature that a planet located at that distance would have but without an atmosphere to retain heat . For Earth it is 255 K , or about -20 ºC. It is our atmosphere that brings that temperature up to the approximately 15ºC global average temperature that we enjoy. There are several planets with equilibrium temperatures very similar to ours. Luyten b, TRAPPIST-1d, and Kepler-1653b would be only a few degrees warmer, while Kepler-1540b would be slightly cooler. But we insist, everything will depend on the atmosphere of each planet.
All the planets we have mentioned so far have similar properties to our own, but they orbit considerably cooler and smaller stars . Since there could be some feature of these stars that makes it difficult for life to arise, let’s find out which planets orbit a Sun – like star. identical to ours. If we replaced the Sun with any of these, we would hardly be able to notice the difference with the naked eye, although perhaps we would notice the accumulated effect of the greater or lesser luminosity of the new star.
Putting it all together, we could have several candidate planets to be the most similar to Earth, although we will stay with two. On the one hand with Teegarden b , which has a similar size, mass and equilibrium temperature, even though it orbits a much smaller star. We would also have Kepler-22b , which, although it has a radius approximately twice that of Earth, has an equilibrium temperature and a year length similar to ours, and orbits a star very similar to the Sun. At present , we do not know of any planet that has all of these Earth-like features , so we have to make do with these partially-like planets.
Petigura, Erik A. et al, 2013, Prevalence of Earth-size planets orbiting Sun-like stars, Proceedings of the National Academy of Sciences. 110 (48), doi:10.1073/pnas.1319909110