The fourth planet - TRAPPIST-1d - orbits in a borderline region at the inner edge of the habitable zone.
"They're too close together now, so they must have migrated to where we see them".
The observations were made while the planets were in transit in front of TRAPPIST-1.
"One of these four could be a water world, one could be an exo-venus, and another could be an exo-Mars".
But at least in the case of one nearby star, scientists are slowly narrowing down the options.
Scans found three planets could be habitable, out of the seven Earth-sized planets orbit the ultracool dwarf star TRAPPIST-1, 40 light-years away from the Earth. We're uncertain whether it has a large atmosphere, an ocean or an ice layer.
The global study was published in the journal Astronomy and Astrophysics. The differences in the light as it passes through the atmosphere can reveal details about its composition. A tiny fraction of the photons will have passed through the planet's atmosphere on their way to Earth.
It's possible, but not easy.
This illustration shows the seven Earth-size planets of TRAPPIST-1. Images taken during this time have more radiation-induced noise and lower resolution, as the telescope shuts down its fine-pointing hardware.
When we finally find life somewhere out there beyond Earth, it'll be at the end of a long search.
He stressed that despite the speculation scientists were still "far from establishing" whether Trappist-1e really does have conditions suitable for life.
The TRAPPIST-1 planets huddle so close to one another that a person standing on the surface of one of these worlds would have a spectacular view of the neighboring planets in the sky.
That doesn't tell us what's in these atmospheres. These planets are incredibly far away, which can make it very hard to study them. For b and c, for example, the authors say the options include "atmospheres dominated by water, nitrogen, or carbon dioxide; tenuous atmospheres composed of a variety of chemical species; and atmospheres dominated by aerosols".
In contrast, Solar System's rocky planets likely formed in the hotter, dryer region closer to the Sun.
The presence of puffy, hydrogen-dominated atmospheres would have indicated that these planets are more likely gaseous worlds like Neptune.
Meanwhile, F, G and H are the coldest planets and are expected to have water "in the form of ice". This is done by a combination of observations.
"It is interesting that the densest planets are not the ones that are the closest to the star, and that the colder planets can not harbor thick atmospheres", said Caroline Dorn, study co-author based at the University of Zurich, Switzerland. Given their mass, we can calculate their density, which tells us a lot about their likely composition. But that has not been possible with TRAPPIST-1, because the star's light is so faint. It took most of 2017 to invent new techniques and run simulations to characterize the planets' compositions. Those variations in orbital timing are used to estimate the planets' masses.
Dr Amaury Triaud, from the University of Birmingham, a leading member of the global team, said: 'Of the seven planets, and of all the exoplanets that have been identified so far, Trappist-1e is the most resembling Earth, when we consider the amount of energy a planet receives from its star, and its density, which reflects its internal composition. The result was an optimized solution that included the masses of all the planets.
In the meantime, however, the Hubble study did produce noteworthy results, which support the terrestrial and potentially habitable nature of planets within the TRAPPIST-1 system. These could be thick atmospheres, water, or an icy shell. That means the water content of the TRAPPIST-1 planets is way greater than the water content of Earth which is just 0.02 percent of our planet's mass. Like 1c, it seems to have a relatively sparse atmosphere. Planet d is right near the limit for a runaway greenhouse, so it could easily have a similar water-rich atmosphere. The amount by which the starlight dims is related to the radius of the planet.
"We're not completely sure water is there, but it's a plausible scenario".