The announcement that seven Earth-sized planets had been discovered 39.5 light years away has been big news here on Earth.he recent additions are in their star’s habitable zone and may harbour liquid water. This led to speculation that the planets could host alien life, or have breathable atmospheres. Habitable or not, the planets have so far proved to bear little resemblance to Earth. Their sun (called TRAPPIST-1 after one of the telescopes that helped discover it) is only 10% of the mass of our sun and is an ultra cool dwarf star. This means that, viewed from the planets, it would be about as dim as Earth’s sunsets. Even stranger, they have very short orbits and seem to be tidally locked, meaning that one side is always facing the sun and the other facing away. While this may have put a few obstacles in life’s development, some believe that the best chances of life are on the improbably named terminators, the twilight regions separating perpetual day and night.
Lots has already been written about these distant worlds, which have fired imaginations on Earth into wild speculation. Time will tell whether this will lead to greater engagement with astronomy and space exploration, but (from the media at least) there seems to be real excitement about what’s out there. Thinking about all this – about stuff 235 trillion miles and its relation to our place in the universe, sentient life’s apparent uniqueness to Earth and mankind’s possible futures is all pretty humbling. Detecting potentially habitable planets far away from our solar system really feels like being on the cusp of another Age of Discovery.
The Age of Discovery usually describes the period from the 15th to 18th centuries when Europeans first became aware of large parts of the Earth’s geography. This includes the trans-Atlantic voyages of Christopher Columbus in the 1490s, the Russian exploration of Siberia and Magellan’s circumnavigation of the globe between 1519 and 1522. Incredible though these achievements were, all of the continents and most of the islands discovered by Europeans had human populations that had been there for hundreds or thousands of years, and who with hindsight would very likely have rather been left undiscovered.
Around the same period a different type of explorer had their sights set on things even grander and further away. By looking at the known planets through telescopes, astronomers were able to improve their understanding of these distant bodies and their motions. With bigger and better telescopes, more planets were discovered. People wondered if our cosmic neighbours could sustain life. Were there trees up there? Animals? Bipeds with opposable thumbs?
So far the answer seems to be no. Since the late 20th century, unmanned probes, together with manned missions to the moon suggest that the Earth aside, the solar system is barren and lifeless. The other planets have such different conditions to the Earth that if life did exist elsewhere, it would need to be very different to life as we know it. This is not to say that life of some sort couldn’t exist elsewhere in our solar system. Given the diversity of life on Earth, beings who evolved elsewhere could look nothing like us, they may not even need water to survive and could be silicon based. They could be so weird as to have a totally different understanding of time, could be hiding in dark matter, or even the laws of physics themselves. Exhilarating though these possibilities are, no direct evidence of extra-terrestrial life has yet been found.
More recently, astronomers have been able to look beyond our solar system. While Isaac Newton and the 16th century Italian philosopher Giordano Bruno both speculated about worlds orbiting other stars, actually exploring the nearest stars for signs of additional planets wasn’t possible until the late 1980s. This is because the amount of light reaching us from even these nearest stars is so miniscule. The first discovery of an exoplanet was confirmed in 1992, with more than 3300 confirmed as of February 2017 and thousands more awaiting confirmation. These come in all shapes and sizes, from Earth sized planets to gas giants larger than Jupiter, orbiting stars similar to our own or pulsars, and even orbiting several stars. Many of these planets are either too big or too far away from their stars to be able to harbour carbon-based life, however some planets, including some of the most recent TRAPPIST-1 additions are inside their star’s habitable zone.
Despite the potential alien-ness of extra-terrestrial life, scientists including NASA’s Kepler programme, have tended to focus on planets that could harbour the type of carbon based life found on Earth, and despite terrestrial life’s diversity (jellyfish, thermo-resistant bacteria, bats etc), all known organisms are made of the same types of organic compounds. To form, these require liquid water, which can only exist in a specific range of temperatures and atmospheric pressures. The habitable zone around a star is the region of space where planets could sustain liquid water providing they had the right type of atmosphere. This is no guarantee of habitable conditions: Mars and Venus are both within our sun’s habitable zone, but Mars’ atmosphere is too thin and Venus’ is too thick. Neither does it preclude the possibility of silicon, methane, dark-matter or the-very-laws-of-physics-themselves based life elsewhere in the universe, with multiple habitable zones based on substances other than water being proposed.
So, what next? In the immediate future, NASA are preparing to launch the James Webb Space Telescope in 2018. “With much greater sensitivity, Webb will be able to detect the chemical fingerprints of water, methane, oxygen, ozone, and other components of a planet’s atmosphere” (NASA website). This will allow scientists to determine whether the planets have Earth-like atmospheres or liquid oceans. At the same time, the hunt for exoplanets continues at an accelerating pace. Potentially habitable systems like TRAPPIST-1 may turn out to be rare, or we may make many similar discoveries over the next few years. Either way, we seem to have crossed some of the technological barriers to explore the universe, albeit from the comfort of Earth.