This episode's coordinates point to a star system in the constellation Dorado, which I have always found to be the most confusing to understand in terms of what its name is supposed to mean. Although it has the same name as a legendary city of gold sought after by Spanish conquistadors in the New World during the early colonial era, it apparently does not reference that. Instead, it's named after the dolphinfish [1], which is better known to people who have lived in Hawaii as mahi-mahi and which doesn't really look like a dolphin, or at least a bottle-nosed dolphin. It's another constellation invented by the Dutch navigators Pieter Dirkszoon Keyser and Frederick de Houtman in the sixteenth century [1] because they wanted to create new constellations to cover what was otherwise blank parts of the southern sky.
Even though the stars within the constellation are relatively indistinct when viewed with the naked eye, this constellation is, to use the technical term, chock full of exciting astronomical objects, the most notable being the Large Magellanic Cloud, but I'm not talking about that today. Instead, I am going to focus on the star system TOI-700. This sounds like it could be the name of some sort of robot from a 1980s sci-fi B movie. However, TOI is an acronym that stands for TESS Object of Interest [2], where TESS is another acronym that stands for Transiting Exoplanet Survey Satellite (and thankfully, TESS does not contain yet another acronym). At this point, you should be able to guess that TOI-700 has one or more exoplanets, and you would be right.
So, before I discuss the exoplanets, let me just describe the star at the center of TOI-700 to begin with. This star is a relatively non-descript red dwarf with a mass of about 0.42 times the mass of the Sun [3] that is located at a distance of (31.12 pc) [4, 5], which is kind of close in terms of stars but still too far for me to use my Star Wars sound effects. It has an apparent magnitude of around 13.1 [6], and it's been appearing in star catalogs for decades, but until exoplanets were discovered orbiting it, it seemed kind of boring.
The exoplanets orbiting TOI-700 were discovered by astronomers using the TESS satellite in a set of observations starting in July 2018, and the first results were announced in a series of papers published in 2020 [3, 7, 8]. The TESS satellite detects exoplanets by looking at the slight periodic dimming that occurs when exoplanets pass between the stars that they are orbiting and the Earth. Detecting stars this way requires the orbits to be aligned in a very special way, but the results can be spectacular when such chance alignments are found.
TOI-700 has a total of four exoplanets, which have the names TOI-700b, c, d, and e. The b planet is the innermost, followed by c, e, and then d. The e planet is out of sequence there because it was discovered a couple of years after the others [9]. These planets orbit the star at distances ranging from 0.068 to 0.16 AU [9] (where 1 AU is the distance from the Sun to the Earth and 0.39 AU is the distance from the Sun to Mercury), so the planets are quite close to their host star. The planets' orbital periods are also quite rapid, ranging from slightly under 10 Earth days to 37.4 Earth days [9]. The b, d, and e planets are all close in size to the size of the Earth, which is rather exciting, while the c planet is 2.6 times larger in diameter [9].
Even though the planets in the TOI-700 system are quite close to the red dwarf, the red dwarf emits around 2% the total energy emitted by the Sun [3], so the planets do not get quite so hot. The b and c planets are still sufficiently close to the red dwarf that they are generally not expected to have Earth-like conditions. However, the d and e exoplanets sit within the habitable zone [3, 9], or in other words, they are at a distance from the red dwarf where the surface temperatures would be similar to the Earth's and where water could exist in a liquid state. In fact, when TOI-700d was discovered, it was the first Earth-sized exoplanet found in the habitable zone of any star observed specifically by the TESS satellite [3], and a lot of astronomers generated a lot of press releases about the discovery and also probably earned a lot of grant money.
However, TOI-700 is not the first star with exoplanets to appear in my podcast series, and I have learned two major points regarding exoplanets. First, almost everyone will have really high hopes that they found an exoplanet that could host life or that is otherwise really unusual compared to what everyone else found (except for the people who found the exoplanet orbiting the white dwarf WD 0806-661B who, as I discussed in episode 44, described the exoplanet as a really really really small brown dwarf). Therefore, it's quite common to see people exclaiming that they found some sort of Earth-like exoplanet orbiting another star but for people later to discover that the exoplanet is not as suitable for life as expected. Second, red dwarfs like TOI-700 often tend to have lots of stellar flares that can ionize or bake off the atmospheres of any Earth-like planet, and this has crushed the hopes of some astronomers who have found exoplanets orbiting other red dwarfs. This could have also been a concern regarding the possibility that TOI-700d and TOI-700e have Earth-like atmospheres. However, the people who discovered the exoplanets orbiting TOI-700 studied the brightness of the star over time very carefully and reached the conclusion that the star did not flare like other red dwarfs [9]. This makes it much more likely that the planets oribiting that star may have atmospheres and may even host life.
One of the cool things that can be done with planets that pass between their host stars and Earth is to perform spectroscopic observations to look for hints of light passing through the planets' atmospheres. Unfortunately, it seems to be technically infeasible to do this with the planets orbiting TOI-700 [3]. Still, it would be interesting to see whether, in a decade or two, someone can build an instrument that can actually detect atmospheres around the exoplanets TOI-700d and TOI-700e.