On February 20th 2017, NASA announced a press conference to discuss a “discovery beyond our solar system”. Two days later they revealed the TRAPPIST-1 system; a series of seven earth-size planets orbiting a star 39 light years from Earth. The announcement of this discovery and the discussion that followed has circled one tantalising question – could the TRAPPIST-1 star system harbour extraterrestrial life?
The scientists at NASA certainly seem excited about the concept. Their search for exoplanets – that is, planets orbiting stars other than our own sun – had until this point yielded only a handful of potentially habitable worlds. Among TRAPPIST-1’s seven planets, however, no less than three have shown this potential, setting a record for the most known earth-like planets orbiting one star.
These worlds were highlighted by the scientists primarily for their location in the so-called ‘habitable zone’. This describes the range of orbit sizes where, dependent on the size of the planets and the star, it is neither too hot nor too cold for life to sustain life.
Artists impression of the surface of TRAPPIST-1f, the fifth planet in the TRAPPIST-1 system Credit: NASA/JPL-Caltech/R. Hurt, T. Pyle
Further investigations by NASA scientists have also yielded promising results. Using precise measurements of the size and mass of the planets, the researchers were able to calculate estimates for the density of each of TRAPPIST-1’s worlds. These density measurements are key to understanding exoplanets as they give us our first insight into their composition.
Of the seven planets in the newly-discovered system, six have been described as ‘rocky’ – that is, more comparable solid planets like Earth and Mars than gas giants like Jupiter and Saturn. The seventh planet, which has the widest orbit and an undetermined mass, has been provisionally described as ‘snowball-like’.
Despite these hopeful indications, there is also a body of evidence which is significantly less inspiring. Firstly, while it’s tempting to imagine TRAPPIST-1 as a distant copy of our own solar system, the absence of two planets is not where the dissimilarities end. The most striking difference between this newly-discovered system and our own is the star which lies at the centre.
The star is classified as an “ultra-cool dwarf”, meaning it is both ten times smaller than our sun and less than half its temperature. While this doesn’t sound like a recipe for warm earth-like planets, the small size of the star is counteracted by the proximity of the planets which orbit it.
NASA’s illustration of the size of TRAPPIST-1. (Credit: NASA/JPL-Caltech/R. Hurt, T. Pyle)
The seven worlds of the TRAPPIST-1 system all orbit between one and five million miles from their star. This means that all seven planets could fit comfortably in the space between the sun and Mercury, with its 58-million-mile orbital distance. While the size of the TRAPPIST-1 system isn’t necessarily a barrier to the formation of life, it certainly represents a significant divergence from the only solar system where we’ve ever observed it.
It’s also important to bear in mind how little is known about the planets of the TRAPPIST-1 system. Despite the array of concept images released by NASA in the wake of the announcement we don’t, in fact, have any idea what the planets look like. The planets were found, or more accurately their existence was inferred, by observing the light emitted by the star they orbit.
This process, known as transit photometry, involved watching the star’s brightness over time and finding dips in luminescence when planets passed in front of it. From this information, NASA scientists extrapolated a range of information, such as their size, mass and orbital distance. What this technique doesn’t reveal, however, are other key factors that determine the habitability of a planet.
Because of this, we still do not know whether any of the TRAPPIST-1 planets contain atmospheres, which are vital for life, or magnetic fields, which can protect life from deadly solar wind. NASA are also not discounting the possibility that some or all the planets may be ‘tidally locked’, meaning that one side may permanently face the sun with the other half perpetually facing away. Conventional wisdom suggests life would be impossible on such a planet, as one half would be too hot for life and the other too cold. More recent evidence, however, has suggested otherwise.
NASA’s idyllic concept art is based almost entirely on speculation. (Credit: NASA/JPL-Caltech/R. Hurt, T. Pyle)
Furthermore, since the announcement of the NASA’s discovery, two pieces of research have poured cold water on hopes of life in the TRAPPIST-1 system. The first, published on March 30th, detected frequent flares emitted from the system’s star. Considering the small orbital distances of the nearby planets the authors feared that these huge releases of energy may disrupt the atmospheres of the planets and that without the protection of strong magnetic fields, life in the system may be impossible.
If that wasn’t disheartening enough, research published on April 6th revealed a new climate model to assess the habitability of exoplanets. The study concluded that only one of the planets, TRAPPIST-1e, was likely to support liquid water. If this planet does not possess a substantial enough magnetic field to weather the flares from its nearby star (something scientists feel is unlikely), all hope for life in TRAPPIST-1 may be lost.
Despite this disappointing news, research into TRAPPIST-1 continues. NASA has announced plans to use its new James Webb Space Telescope, launching in 2018, to search for key atmospheric components such as oxygen and water in the system.
The increased sensitivity of the Webb telescope will also allow the surface temperature and pressure of the planets to be measured, answering yet more questions about the habitability of the system. Until then, however, the hospitability of the TRAPPIST-1 system remains very much in question.