An artist's view of planets transiting a red dwarf star in the TRAPPIST-1 system. NASA/ESA/STScI

In early May, a team of astronomers led by MIT announced that they had discovered three Earth-like planets orbiting a small dwarf star, known as TRAPPIST-1, only 40 light years away. The planets are considered the most likely worlds to find life outside our solar system due to their size, moderate temperatures, and their short distance from us which makes them easy to observe. Now, the same team of scientists has announced that the two innermost planets of the system have rocky surfaces, increasing the likelihood that they could harbor life as we know it. A paper detailing the new findings was published today in Nature.

"Now we can say that these planets are rocky. Now the question is, what kind of atmosphere do they have?" said Julien de Wit, a postdoctoral planetary science researcher at MIT, in a press release. "The plausible scenarios include something like Venus, where the atmosphere is dominated by carbon dioxide, or an Earth-like atmosphere with heavy clouds, or even something like Mars with a depleted atmosphere. The next step is to try to disentangle all these possible scenarios that exist for these terrestrial planets."

"Now the question is, what kind of atmosphere do they have?"

Because the planets are so close, and because they orbit a dim dwarf star, they are perfectly suited for spectral analysis which could determine the composition of their atmospheres. Certain elemental ratios could be indicative of life. For example, large amounts of oxygen—more than chemical equilibrium would allow—could suggest some sort of plant life engaging in photosynthesis, much like on Earth.

A rare opportunity to study the planets had the researchers writing a last minute plea to point the Hubble Space Telescope at TRAPPIST-1. The researchers realized only two weeks in advance that a double transit would occur—the two innermost planets of the system would pass in front of the star at the same time.

"We thought, maybe we could see if people at Hubble would give us time to do this observation, so we wrote the proposal in less than 24 hours, sent it out, and it was reviewed immediately," said de Wit. "Now for the first time we have spectroscopic observations of a double transit, which allows us to get insight on the atmosphere of both planets at the same time."

Not only were observations from Hubble able to confirm that the two innermost planets, TRAPPIST-1b and c, were rocky, but also that they have compact atmospheres similar to what you might find on Earth, Venus, or Mars. The changes in starlight over a specific wavelength as the planets transited did not vary significantly, suggesting that the two planets' atmospheres are compact and dense, unlike the voluminous but sparse atmosphere of a gas giant like Jupiter.

"The data turned out to be pristine, absolutely perfect."

"The data turned out to be pristine, absolutely perfect, and the observations were the best that we could have expected," said de Wit. "The force was certainly with us."

Looking forward, the research team has formed a consortium called SPECULOOS (Search for habitable Planets Eclipsing ULtra-cOOl Stars) that hopes to raise money to build new infrared telescopes like TRAPPIST (TRAnsiting Planets and PlanetesImals Small Telescope) to search dwarf stars for orbiting planets. The discovery of the three planets around TRAPPIST-1 in May has many scientists thinking that Earth-like planets around ultra-cool dwarf stars might be in abundance, we just don't have the tools to effectively search for them.

Once we find more planets orbiting starts like TRAPPIST-1, future telescopes like the James Webb Space Telescope, scheduled for launch in 2018, could help determine the precise composition of the planets' atmospheres to look for clues that life is impacting the abundance of certain elements.

Will our first discovery of alien life be in our own solar system, or clinging to a rock orbiting another star like TRAPPIST-1? It's anyone's bet, but you better believe planetary scientists are frantically searching.

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