Yesterday NASA announced the discovery of seven Earth-sized exoplanets orbiting a small, dim star 40 light years from Earth. Trappist-1 is an unprecedented discovery, and is sure to keep astronomers busy for decades to come, but also offers one of our best hopes in the hunt for extra-terrestrial life.
Located in the Aquarius constellation, the exoplanet system contains three planets in the habitable zone, of which at least two are thought to have a rocky surface. And while this doesn’t guarantee the existence of life in the system, it does make it worthy of further investigation.
“Three of these planets are in the habitable zone where liquid water can pool on the surface. In fact, with the right atmospheric conditions there could be water on any of these planets,” said Thomas Zurbuchen, associate administrator of NASA’s Science Mission Directorate in Washington.
Over the next decade scientists will be performing numerous follow-up studies, with the soon-to-be-launched James Webb Space Telescope enabling scientists to detect evidence of water, methane, oxygen and other vital building blocks of life when it comes online in 2018.
“These planets are among the best of all the planets we know to follow up, to see the atmospheres, and also to look at biosignatures – if there are any,” added Zurbuchen.
“The discovery gives us a hint that finding a second Earth is not just a matter of if, but when.”
Under different suns
Trappist-1’s star is quite different from our own Sun, meaning that any life that has evolved in its presence would be quite unlike that of Earth.
Most significantly, Trappist-1 is a red dwarf star, a class of stars also known as M-dwarfs that are increasingly being targeted in the search for life.
This M-dwarf is considerably smaller and burns at a lower temperature than our solar system’s star, and is smaller and cooler than most other M-dwarfs, hence the ultracool classification. As a result, liquid water can exist on planets orbiting very close to it; the seven planets hug their star in tight orbits, all of which are closer than our innermost planet Mercury’s orbit of the sun.
This also means that the planets orbit considerably closer to each other than we do with our own planetary neighbours. If you were standing on the surface of one of the Trappist-1 planets, your planetary neighbour on some days would hang larger than our own Moon in the sky, and might be close enough to see its mountain ranges or cloud cover.
The sun would also be a far greater presence in the sky, looming six times larger than our own.
This would also mean trips between different planets in the system could take just a couple of days, potentially allowing if not life in the system then future humans to hop across Trappist-1.
A year a week
Because the planets are so much closer to their sun, their years are very different to our own, ranging from 1.5 days for the closest planet to the star to 20 days for the farthest.
For the three planets in the habitable zone, snappily named Trappist-1e, f and g, years are 6.1 days, 9.2 days and 12.4 days long respectively.
What impact, if any, that could have on life is unclear, but it does have the potential to affect how life evolves; on Earth many forms of life have seasonal responses that are influenced by the changes and length of our year.
Forever day, eternal night
NASA also believes that the planets may be tidally locked, meaning that one side of each is always facing the sun. This would result in life on the planets either eternally basking in daylight, or permanently shrouded in darkness.
It would also make for a very different weather system on each planet, with extreme temperature changes, and strong winds over the terminator – the line between day and night.
This could mean that life would require a certain atmosphere to be present for it to survive, in order to transport heat and moderate the overall climate, which is something that astronomers will know more about once the James Webb space telescope launches in 2018.
However, the wavelength of light Trappist-1’s star is supplying is also different to our own sun. This will result in a different hue, with a duskier red-orange daylight.
This would affect the wavelengths of light that life would be exposed to, and so would have an impact on how biological systems evolved in response. On Earth, plants photosynthesise best at specific wavelengths and have evolved to reflect unwanted green light from the Sun, giving them their colour. But on the Trappist-1 planets there will be a different spectrum of light, requiring any plants to adapt differently to their environment.
As a result, plants on Trappist-1’s planets could have orange and black foliage rather than our own green.
The hunt is on
Now that the world knows about the existence of the planets, scientists are scrambling to learn more about them. However, with no ability to send anything directly, there are limitations on what we can currently learn, and the scientists are keen to stress that any life found is highly unlikely to be sentient.
“I’m just talking about slime here – it’s far easier to evolve than sentient beings.” said Victoria Meadows of the University of Washington, the principal investigator for the NASA Astrobiology Institute’s Virtual Planetary Laboratory. “The majority of life we find out there is likely to be single cell, relatively primitive life.”
However, when the James Webb Space Telescope (JWST) finally comes online next year, scientists will be able to start looking for an atmosphere.
The majority of life we find out there is likely to be single cell, relatively primitive life.
“We will look at the atmosphere for gases that do not belong – gases that might be attributed to life,” said Sara Seager, a professor of planetary science and physics at MIT, in a Reddit AMA. “We will not know if the gases are produced by microbial life or by intelligent alien species.”
Beyond that, we will need to build more sophisticated equipment if we are to determine what the flora and fauna of Trappist-1 is really like.
“In order to see vegetation and any other surface features (e.g. oceans, continents), we’ll need future telescopes beyond JWST that will be able to directly image exoplanets,” added Giada Arney, an astrobiologist at NASA Goddard Space Flight Center.
“We’ll need farther future technology that may become available in the coming decades that will allow us to block out the star’s light and observe the planets directly.”