by Ludwig Scheibe (TU Berlin), November 2024
At the lower rung of the planetary size ladder, we have terrestrial planets: worlds smaller than about 1.5 times Earth’s radius. In our Solar System, the four inner planets belong to this class: Mercury, Venus, Earth, and Mars, of which Earth is the largest. Like its siblings in our neighborhood, terrestrial exoplanets consist mostly of rock, with a possible iron core, and only a very thin gaseous atmosphere compared to Jupiter– or Neptune-like planets, if they even have any atmosphere at all. As small planets are the most difficult to find and explore, we have only found a few in this class of worlds, but of course due to their similar size and mass to Earth, they are particularly interesting as research objects.
Artist’s impression of ultra-hot exoplanet GJ-367 b: two-thirds as large as Earth, and so close to its star that the surface could easily be over 1300°C. Credit: SPP 1992 / Patricia Klein
Highly different environments
These planets may be about the same size as Earth, but that does not mean that they show the same conditions as our home world. For an example, we only have to look towards Venus right here in our Solar System: It almost mirrors Earth in size and mass, but as opposed to our home, it has a very dense and hot atmosphere made of carbon dioxide with clouds of sulfuric acid, that has a surface pressure 92 times that of Earth normal conditions and a temperature of 460°C. For exoplanets, conditions can be even more extreme: GJ-367 b for example, pictured above, orbits its star in only 8 hours is irradiated so strongly that the side that faces the star has a temperature upwards of 1300°C and might very well be completely molten.
So it is good to keep in mind, if an article or press message speaks of “terrestrial” or “Earth-like” planets, that only means that they have roughly the same size as our planets, and not that it looks just the same or that even that it has life.
Depending on the conditions, an Earth-sized planet can look very different. One factor is temperature: is it an ultra-hot lava planet (left), a moderate twin of Earth’s environment (middle), or a cold and ice-covered world (right)? Artist’s impressions, Credit: SPP 1992/Patricia Klein
The search for the second Earth
So where do we stand with respect to a “second Earth”? Have we found the mirror image of our home planet?
In the course of the past decades, we have found some Earth-sized planet in the habitable Zone around its star. The most well-known example is probably TRAPPIST-1, discovered in 2017, which hosts seven planets known to us. All of them are between 0.8 and 1.3 as big as Earth, and three are in the habitable zone. But there is an important difference: The host star is a red dwarf. That means it is only about one tenth the size of the sun, and only half as warm, which makes it fainter than the Sun by several orders of magnitude.All seven known TRAPPIST-1 planets have orbits that are narrower than Mercury’s orbit around the sun by far. Furthermore, it is quite likely that these planets are tidally locked, meaning that they always face their star with the same side. All of that makes the conditions there quite different than Earth, and it is still unclear if the middle planets have any atmosphere at all, much less if it in any way resembles ours.
Artistic representations of the planets in the TRAPPIST-1 system. The planet sizes are to scale. As a comparison the inner planets of the Solar System. The corresponding habitable zones are colored in green. Credit: NASA/JPL-Caltech
Regarding an actual second Earth – so a terrestrial planet orbiting a star of the same type as the Sun, with the same distance as Earth: Such a twin has not been found so far. But that doesn’t mean that there are none out there. Such a twin would need a year for a full orbit, same as Earth, which means you would have to observe the star for at least two years to be sure to get indications for a planet with the transit and radial velocity methods. Furthermore, you would have to measure the starlight extremely precisely, to find such a small planet.
One of the original goals of the Kepler mission was to determine how many of these second Earth candidates there are. But while this telescope has unquestionably found thousands of new plents and advanced our understanding of planet occurrence considerably, it could not answer this specific question. Now, the PLATO mission, a new space telescope by ESA with a launch slated for 2026, is expected to tell us more. It is designed specifically for the search for such Earth twins and will hopefully give us an answer on how frequently such worlds can be found.
