Kepler-138 is a rather dim red dwarf about 0.57 times the mass of the Sun [1] located about 218.9 light years (67.1 pc) from the Earth [2, 3] in the constellation Lyra. People who know a little bit of astronomy might guess that the star is named after Johannes Kepler, the seventeenth century astronomer who developed the mathematics describing how planets orbit the Sun. That guess would be incorrect. This star is not named after him, or at least it's not named directly after him. People who know a little bit more about astronomy might guess that the star is named after the Kepler spacecraft, which was named after Johannes Kepler and which has been used to search for exoplanets orbiting other stars. That guess would be correct. Given this information, some people might guess that the star has an exoplanet orbiting it. That guess would also be correct, although, in a way, it's only partly correct.
So, to backtrack a little bit, the Kepler spacecraft was launched in 2009, and it was designed very specifically to search for exoplanets [4]. Since directly imaging planets in orbit around other stars is extremely hard, Kepler used a different technique to find exoplanets. It monitored the brightnesses of stars very closely and waited for the brightnesses of these stars to drop slightly, which would indicate that an exoplanet was passing between the star and the Earth. The spacecraft had to watch these stars for a long time so that it could see that the stars were dimming in brightness in a periodic way that would be consistent with an exoplanet orbiting the star. After all, stars could vary in brightness for other reasons that have nothing to do with exoplanets.
One of the disadvantages of this technique is that, to detect an exoplanet orbiting a star, the exoplanet has to pass directly between the Earth and the star that the exoplanet is orbiting. If the exoplanet's orbit looks tilted as seen from Earth, the exoplanet will not be seen. For Kepler to find an exoplanet, the exoplanet also needed an orbital period that was shorter that the Kepler spacecraft's lifetime. The spacecraft was shut down in 2018, so if it was in another star system looking at our Sun, it would have found the Earth and the other inner planets, all of which have orbital periods much shorter than 8 years, but it would not have found any of the gas giants, which have much longer orbital periods. However, the key advantage that Kepler had was that it could easily monitor the brightnesses of lots and lots of stars all at once, so it was quite capable of finding exoplanets. At the time of this recording, Kepler data have been used to identify 2778 confirmed exoplanets[5].
So, like I indicated earlier, it was, in a way, partly correct to say that an exoplanet was found orbiting Kepler-138. In fact, four exoplanets have been found orbiting Kepler-138 [6]. Three of these exoplanets were discovered within the first couple of years of the spacecraft's operation, and the first paper mentioning these three exoplanets was published in 2011 [7]. The fourth was discovered in an analysis published in 2023 [6]. These exoplanets have been given the uncreative designations Kepler-138b, c, d, and e. For your information, a lot of my statistics about these exoplanets comes from a paper published in 2023 by Caroline Piaulet et al. in the journal Nature Astronomy [6].
Kepler-138b is the innermost planet. It's orbit has a radius of of 0.0753 AU (where 1 AU is the radius of the Earth's orbit), and although, Kepler-138b is closer to its host star than Mercury is to the Sun, Kepler-138b does not get as hot, although the surface temperature is still 180 degrees Celsius [6]. The mass of Kepler-138b is 0.07 times the mass of the Earth, which would make it about the same mass as Mars [6]. Kepler-138b also has a relatively low density of 1.7 g/cm3 [6]. For context, the density of the Earth is about 5.5 g/cm3 [8]. The low density of Kepler-138b may indicate that it contains a lot of water or some other type of volatile molecules, which would either have to be located underneath the planet's surface or which would form some sort of steay atmosphere [9].
Kepler-138c is the second planet in the system. The exoplanet's orbit has a radius of 0.0913 AU, which, again, places the exoplanet close to its host star, but the temperatures only reach about 140 degrees Celsius [6]. It has a mass about 2.3 times the mass of the Earth, and while Kepler-138c has a density of 3.6 g/cm3, that density still seems low for a planet twice the size of the Earth, which implies that the planet has a lot of water or other low-density volatile molecules [6]. However, the exoplanet's surface temperature is sufficiently high that the water and the other stuff would potentially have to be buried underneath the surface of the planet or located within a thick atmosphere of steam [6].
Kepler-138d is the third planet in the system. This is where things start to get interesting. This planet orbits at a distance of 0.129 AU, which in our Solar System would be a really roasty place but in a system with a red dwarf makes it nearly habitable [6]. The average surface temperature is around 72 degrees Celsius, which is still a bit high, but not high enough to boil water [6]. The mass is about 2.1 times the mass of the Earth, and the density is 3.6 g/cm3, which again seems low for a planet larger than Earth [6]. This combination of temperatures below boiling and low density indicates that Kepler-138d could be a planet with a lot of liquid water on its surface, which might form a deep ocean covering a solid core of rock and iron [6]. It would be a water world as described by Caroline Piaulet [6]. Of course, after using that phrase, we all have to think of the movie from the 1990s, but once we get past that joke, this is a truly amazing discovery. To me, it seems like the type of place that could host some form of aquatic life. It would be literally like something out of science fiction except for the absence of Kevin Costner on a trimoran.
Kepler-138e is the outermost planet in the system, or at least the outermost planet that we know about, is the only exoplanet that does not pass between us and the star Kepler-138 [6]. Its orbit appears to be tilted slightly compared to the other planets in the system. The existence of Kepler-138e was inferred by the fact that the orbits of the other planets could not be accurately modelled unless a fourth planet was included. Because Kepler-138e does not pass in front of the star, it was more difficult to measure the exoplanet's properties. However, it seems to have an orbital radius of 0.180 AU, a mass of roughly 0.42 times the mass of the Earth, and, most interestingly, a surface temperature of about 19 degrees Celsius [6], which may mean that Kepler-138e is capable of supporting life.
Overall, this is an exciting planetary system. After all, how many other astronomical objects can be described as "water worlds"? OK, I don't know if referencing a high-budget 1990s action film with mixed reviews is the best thing to do; maybe using the phrase "ocean planet", which sounds like it could be a David Attenborough documentary series, would have been better. Still, the planets in this system have significantly changed our assumptions about the types of exoplanets that might be orbiting other stars.