Thanks to the massive amount of exoplanets discovered by the Kepler mission, we now have a good idea of what types of planets are out there, where they are. and how common are the different types. What we lack is a good sense of what this implies in terms of the conditions on the planet itself. Kepler can tell us how big a planet is, but he does not know what the planet is made of. And planets in the "habitable zone" can be anything, from a hell to a frozen rock.
The Transiting Exoplanet Survey Satellite (or TESS) was started with the intention of helping us to find out how exoplanets actually are. TESS was developed to identify planets orbiting bright stars relatively close to the earth, conditions that will allow their composition, and possibly their atmosphere, to be determined.
There is currently a conference dedicated to the description of some of these planets dedicated to the first discoveries made with TESS. These discoveries include a three-planet system that seems to be perfectly positioned to test all our exoplanet characterization techniques.
What is it about?
Both Kepler and TESS identify planets using the so-called transit method. This works for systems in which the planets circle in a plane that brings them between their star of hosts and the earth. The planet blocks a small portion of the starlight that we see from Earth (or from nearby orbits). If these light breaks occur regularly, they can detect an orbit of the star.
This says something about the planet. The frequency of burglaries in the light of the star tells how long an orbit lasts, and how far the planet is from its host star. This, along with the brightness of the host star, tells us how much incident light the planet receives, which affects its temperature. (The distance range in which the temperatures coincide with liquid water is called habitable zone.) And we can use it together with the amount of blocked light to find out how big the planet is.
But that leaves a lot of important questions unanswered.
Take a planet that seems to be bigger than Earth. It could be rocky like the earth and classified in the super-earth category. It could also have a dense core surrounded by a dense, gaseous atmosphere, making it a mini Neptune. Or it could be water-dominated, resulting in a water or ice giant, depending on where it is located.
The last problem is not as clear as it seems. The temperature of a planet depends in part on its distance from the host star (and its brightness). Part of this light is reflected by the surface of the planet and the clouds in its atmosphere. And some of the energy the planet absorbs is trapped in the atmosphere by greenhouse gases (including carbon dioxide).
The composition of the planet and the content of its atmosphere play a major role in influencing its temperature. At a certain distance from a star, it is often possible for these factors to make the difference between a frozen body and a body hot enough to boil its oceans.
So to really understand other planets and their potential to support life, we need to understand what they are made of and what their atmosphere looks like. And although TESS itself does not answer these questions, it should find planets where other instruments could.
Find things to look at
Fortunately, there are ways to find out. For example, if we know the size of a planet and its mass, we know how dense it is and can draw conclusions about its composition. One way is to find out how much a planet is pulling at its star as it moves in its orbit. This drag creates small Doppler shifts in the star's light. This shift allows us to determine the force that the planet exerts on the star, and thus its mass.
Alternatively, planetary orbits, when packed tightly, exert gravitational effects on one another: they accelerate or slow each other's orbits. These transit timing variations can be registered over time and integrated into models that provide plausible estimates of planetary mass.
The transit method can also give us a sense of what's in the planet's atmosphere. As it passes in front of its host star, a small percentage of starlight is absorbed by the gases in its atmosphere, creating a signature that can reveal the identity of those gases. While this tiny signal is flooded with noise in a single pass, the observation of multiple passes may possibly overcome this limitation.
However, all of this initially requires a little light, which means that there is a bright, relatively close star. And we would have to map several orbits, which means that the planet in question must orbit relatively close to its host star.
It's exactly these things TESS is supposed to record.
The New System  The new three-planet system is called TOI-270 and is about 75 light-years from Earth. The star in the middle of the system is a red dwarf, a little less than half the size of the sun. Despite its small size, it is brighter than most nearby stars we know to host planets. And – critically – it's stable. This means that the variations in the light of the star are minimal and less likely to hinder the attempt to reveal subtle changes caused by the orbiting planets.
The three planets have orbital periods of 3.4 days, 5.7 days and 11.4 days. The ratio between these periods can be expressed as the ratio of integers, a feature called "orbital resonance."
These resonances tend to stabilize the orbits and prevent the planetary interactions from throwing one of them out of the system or sending a dive into the stern. Based on the size of the planets, the trio consists of a super-earth as the innermost planet, while the two outer planets are slightly larger and fall into the class of sub-Neptune.
Currently we have only enough observations from the TOI-270 system to confirm the existence of the three planets. However, orbit simulations indicate that a large range of orbital eccentricities in the system is stable. Therefore, detailed observations are needed to determine the exact details of the orbits.
However, since the longest orbit is less than 12 days, that's not so annoying. Once the orbits are found out, the planets are close enough to each other to cause transit timing variations, giving us an opportunity to determine the mass of the planets. They are also large enough and close to the star to shift it slightly during the orbit, creating Doppler shifts that allow independent measurement of the mass.
The two outer planets, which are expected to have significant atmospheres, are major candidates for studies of their composition. The authors estimate that the James Webb Space Telescope may have a view of the system for more than half a year and can read the atmospheric signals for both planets.
What will we probably find? All three planets are extremely hot, and only the outermost planet can possibly absorb some liquid water. However, it is also on the verge of evaporation of water into the atmosphere; The authors assume that moderate conditions on the moons are more likely. The interactions between the three planets also make it likely that everyone is bound to the host star, which can lead to a cold opposite side of the planet and a side facing the back star.
But even if this is not the case One problem here is that the system is definitely worth a careful look. If we have the kind of telescope hardware we can use to detect atmospheres, we want to test them in a place where it probably works, and where we can fix the inevitable problems that will arise in the analysis. There will also be a learning process with the giant telescopes and James Webb, which will be necessary for this work. This makes TOI-270 an important discovery as it provides the perfect test conditions for the characterization of exoplanets.
Nature Astronomy 2019. DOI: 10.1038 / s41550-019-0845-5 (About DOIs).