Thanks to the vast array of exoplanets discovered by the Kepler mission, we now have a good idea of what kinds of planets are where they orbit, ie. and how often are the different types What we miss is a good sense of what it means in terms of the conditions of the planets themselves. Kepler can tell us how big the planet is, but he doesn't know what the planet is made of. And the planets in the "habitable zone" around the stars can be reconciled with everything from burning hell to frozen rock.
The Transiting Satellite Research Exoplanet (or TESS) was launched with the intention of helping us to understand what exoplanets actually are. as. TESS is designed to identify planets orbiting bright stars relatively close to Earth, conditions that should allow subsequent observations to understand their composition and potentially those of their atmosphere.
A conference is currently underway to describe some of the first discoveries made with the help of TESS. These discoveries include a triplanetary system that seems perfectly positioned to test all of our exoplanet characterization techniques.
What are we looking at?
Both Kepler and TESS identify planets using the transit method. This works for systems in which planets orbit in a plane that drives them between the host star and Earth. As it happens, the planet blocks a small part of the starlight we see from Earth (or nearby orbits). If these drops in the light happen with regularity, they diagnose something about the orbit of the star.
This tells us something about the planet. The frequency of immersion in the light of a star shows us how long it takes to orbit, which shows us how far the planet is from its receiving star. This, combined with the brightness of the receiving star, shows us how much light the planet receives, which will affect its temperature. (The range of distances at which temperatures correspond to liquid water is called the habitable zone.) And we can use that, along with how much light is blocked, to understand how big the planet is.
But this leaves many important questions unanswered.
Take a planet that looks bigger than Earth. It can be rocky like Earth, placing it in the super Earth category. But there may also be a dense core surrounded by a thick, gaseous atmosphere that makes it a mini-Neptune. Or it can be dominated by water, leading to a water or ice giant, depending on where it orbits.
And the last problem is not as clear as it seems. The temperature of the planet depends in part on its distance from the host star (as well as the brightness of that star). Part of this light will be reflected by the surface of the planet and all the clouds present in its atmosphere. And some of the energy absorbed by the planet will be trapped in the atmosphere by greenhouse gases (including, yes, carbon dioxide).
Thus, the composition of the planet and the content of its atmosphere play a huge role in the influence of its temperature. At a given distance from a star, it is often possible for these factors to distinguish between a frozen body and one that is hot enough to boil from its oceans.
So to really understand other planets and their potential to sustain life, we need to understand what they are made of and how their atmosphere looks. And while TESS itself does not answer these questions, it is intended to find planets where other instruments could be used.
Finding Things to Consider
Fortunately, there are ways to understand these things. For example, knowing the size of the planet and its mass tells us its density, which in turn allows us to draw conclusions about its composition. One option is to figure out how much a planet is dragging on its star-host as it moves in its orbit. This pull creates small Doppler shifts in the light coming from the star. This displacement allows us to understand the force exerted on the star by the planet and therefore its mass.
Alternatively, if the orbits of the planets are packed tightly enough, they exert a gravitational influence on each other: they accelerate or slow down each other's orbits. These variations in transit times can be recorded over time and incorporated into models that will provide plausible estimates of the mass of the planets.
The transit method also has the potential to give us an idea of what is in the atmosphere of the planet. As it passes in front of the host star, a small percentage of the starlight will be 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 transit, the observation of multiple transits can eventually overcome this limitation.
However, all of this requires quite a bit of light to get started with what it means to be a bright, relatively close star, star.
This is exactly what TESS is meant to take.
The new system
The new triplanetary system is called the TOI-270 and is about 75 light-years from Earth. The star at the center of the system is a red dwarf, slightly less than half the size of the sun. Despite its small size, it is brighter than most of the nearby stars we know about the host planets. And – critically – it is stable. This means that variants in light of the star are minimal and less likely to try to find the subtle changes caused by its orbiting planets.
The three planets have orbital periods of 3.4 days, 5.7 days, and 11.4 days. The relation between these periods can be expressed as the ratios of integers, a characteristic called "orbital resonance".
These resonances tend to stabilize the orbits, holding the interactions of planets to eject one of them from the system or send one dive into the star. Based on the size of the planets, the trio consists of the super Earth as the innermost planet, while the two outer planets are slightly larger and fall into a class called the Neptune.
At present, we have only sufficient observations of the TOI-270 system to confirm the existence of the three planets. Orbital simulations, however, suggest that a wide range of orbital eccentricities would be stable in the system, so extended observations would be required to determine the exact details of the orbits.
But since the longest orbit is less than 12 days, it's not that heavy. Once the orbits are understood, the planets are close enough to each other to cause variations in transit time, providing us with one way to get the masses of the planets. They are also large enough and close enough to the star to drag it around it as it orbits, creating Doppler shifts that would allow the mass to be measured independently.
The two outer planets, which are expected to have a significant atmosphere, are the main candidates for studies of their composition. The authors estimate that the James Web Space Telescope will eventually have an overview of the system for more than half a year and must be able to select atmospheric signals for both planets.
What are we likely to find? All three planets are extremely hot, with only the outermost planet able to hold liquid water. However, it is also at the end of boiling any water into the atmosphere; the authors suggest that moderate conditions are more likely for all moons to be found. The interactions between the three planets also mean that each of them is likely to be well attached to the host star, which can lead to a frigid farther side of the planet and a side facing the blazing star.
But even if habitation is not a problem here, the system is definitely worth considering. When we have the hardware of a telescope that will allow us to detect the atmosphere, we will want to test it for something that is likely to work and where we can eliminate the inevitable problems that will arise in the analysis. There will also be a training process with the giant telescopes and James Webb that will be required for this job. This makes the TOI-270 an important breakthrough as it provides the perfect test conditions for exoplanet characterization.
Nature Astronomy 2019. DOI: 10.1038 / s41550-019-0845-5 (About DOIs).