Exoplanets – the “other Earths” that could bear life in the universe

The recent discovery of Earth-like planets close to home is reshaping our understanding of the worlds beyond the solar system.

Sign Up

Get the New Statesman's Morning Call email.

Update 11.5.16: New data from Nasa's Kepler Space Telescope has confirmed the discovery of an additional 1284 exoplanets, doubling the number previously found by Kepler. Analysis of the 4302 exoplanet candidates found by Kepler in July 2015 lead to the confirmation, with 9 of the 1284 thought to be seated in the habitable zone. The findings were published in the Astrophysics Journal.

Throughout history, civilisations have bridged the gap between consciousness and cosmos with imagination. Aristotelian, geocentric models of the universe shaped our worldview and gave rise to a form of cosmic navel gazing – in which humanity placed itself at the heart of the universe while everything else danced in orbit around it, both literally and figuratively speaking.

This self-centred view rapidly diminished with the dawn of the telescope in 1608 which enabled Galileo to get a closer look at the light emanating from the darkness one year later. And the rest, well, the rest is history. His observations of Jupiter’s moons circling the gas giant drastically reconfigured everyone’s imagining of Earth as the centrepiece of the sky.

The resulting shift in perspective accelerated society towards the development of empirically grounded views of the universe. Nineteenth-century astronomer William Herschel used a wooden telescope to identify what he thought to be star-like “nebulae”; Edwin Hubble (the American astronomer whose name is now synonymous with the discoveries of NASA’s flagship telescope) later went on to realise those nebulae were actually galaxies – systems of gravitationally-bound stars. Billions of stars.

The realisation solidified the notion that Earth occupies a space far from the centre of its own Milky Way galaxy, let alone the Universe. In fact, astronomer Carl Sagan located our planetary postcode as somewhere in the “galactic boondocks”. With time, astronomers have come to understand that the vast majority of stars sprayed across the sky are most probably hosts to other planets, known specifically as exoplanets.

Exoplanets, or extrasolar planets, are defined as planets existing beyond our own solar system – planets which orbit a star other than the sun. As a result of exoplanet discovery, the imagination that humans have continuously applied to the cosmos throughout time has permeated into the more detailed, deeper field picture of space that has now been formed with the inking in of exoplanets. Questions have been raised about the potential likelihood of intelligent life existing beyond the sphere of Earth, whether we can communicate with said intelligent life, and, if so, would there be any chance of physically reaching them.

And rightly so. We should be asking such questions. The vastness of space never ceases to expand, and the thriving of life on our very own speck makes it highly likely that exoplanets are out there also populated with intelligent life. Yet our probing beyond the solar system’s perimeters so far has yielded very little evidence of this. Fermi’s paradox – named after physicist Enrico Fermi – posits the puzzling conundrum and contradiction in the lack of evidence for extra-terrestrial life, in the face of a universe whose physical expanse over a 14bn year time period would make it highly probable that life exists elsewhere.

However, recent discoveries of Earth-like exoplanets have rekindled the passion to keep searching. NASA’s Kepler spacecraft, assigned the mission of finding Earth-sized exoplanets in our region of the Milky Way, has confirmed the existence of 1041 exoplanets since its launch in 2009, with many more on the brink of confirmation.

Close to home

Of those, 12 have been found to have some of the attributes required to potentially harbour life, such as having a size less than twice that of Earth, and a location in the “habitable zone”. This zone is essentially the distance from a star where conditions would allow liquid water to run on the surface of an orbiting planet. Planets have been found in this zone before, but have revealed themselves to be far larger than our humbly sized home. Exoplanet studies has grown over the past few years, and this month, scientists discovered exoplanets much closer to home.

Describing their findings in Nature, astronomers from the University of Liège in Belgium revealed details about the discovery of three potentially habitable exoplanets – the key term here being “potentially habitable”. The planets were discovered using the Transiting Planets and Planetesimals Small Telescope (TRAPPIST), installed at Cerro Paranal in Chile’s Atacama Desert. But what’s the fuss about these planets? And how do they compare to those discovered by Kepler?

Firstly, there’s the factor of distance. Kepler has found exoplanets in the range of hundreds to thousands of light years away from Earth. Most notably, Kepler 186-f - the first Earth-sized planet found in a habitable zone – is 490 light years away from us. For the purposes of perspective, the most distant object in the universe found so far is the galaxy named GN-z11, which is an unfathomable 13.4bn light years away. In comparison to GN-z11, Kepler’s exoplanets seem within our reach. What makes TRAPPIST’s exoplanets distinct, then, is their closeness in proximity to Earth, even when compared to Kepler’s.

The three planets named TRAPPIST-1b, 1c and 1d were found at a distance of 40 light years – around 12 times closer to Earth than Kepler 186-f. The planets were found orbiting a tiny star about the size of Jupiter called TRAPPIST-1, making it an ultra-cool dwarf star (the smallest, coolest known stars). TRAPPIST-1b and TRAPPIST-1c are the planets closest in orbit to the star, whirling around with a complete orbit time just shy of three days. Disappointingly though, this positions them a little too close to the star to harbour life. TRAPPIST-1d, however, may harbour life. It’s found in the habitable zone and is very similar in size to Earth.

A promising candidate then, in the search for reachable Earth-like planets – until the reality of light year distances sinks in. A light year is the distance light travels in one year, and given that light is the fastest moving entity in the universe (it travels at 186000 miles per second), the planets orbiting the ultra-cool dwarf star 40 light years away would put them at an astonishing distance of roughly 240tn miles from Earth. Unless the necessary technology is developed anytime soon, daydreams of reaching these planets should quickly be dismissed. So what can we derive from the discovery?

Life on other earths?

Michael Gillon, lead author of the research, believes that the closeness of the planets will help root out key biomarkers that will reveal the compositions of their atmospheres. This is pivotal because data about oxygen and methane levels, for example, can indicate signs of life. And that would mark a significant step forward. The far-flung distances of exoplanets found in the past have made it tough to accurately determine concentrations and levels of these biomarkers.

Here’s what is known so far about the environments of the TRAPPIST-1 planets. The small size of the TRAPPIST-1 star means that the planets are tidally-locked in their orbit. This means that just like the moon, the planets are locked in a fixed position – not rotating on an axis like Earth, not exposing a side of the planets to the star.

This would inevitably mean that the environments on TRAPPIST-1b, 1c and 1d would be entirely different from the environments on Earth. “The problem for habitability is that one face could be super-hot and the other super-cold, so cold that the atmosphere could freeze and collapse,” Gillon explained to Motherboard. The dissimilarity of these planets’ environment to Earth’s very own climate raises doubts about the possibility of life.

However, in the face of fading optimism, we shouldn’t lose hope. The research team believes that approximately 15 per cent of stars near our solar system are similar to TRAPPIST-1, opening up the possibility to systems accommodating Earth-like exoplanets, in both size and nature. NASA’s launch of the James Webb Space Telescope in 2018 will mainline increasingly detailed information to us about the TRAPPIST-1 system, as well as “Earth’s twin”, Kepler 186-f.

The detection of TRAPPIST-1’s planets this month and Kepler’s discoveries since its inception may not have given us a clear insight into life beyond our planet, but it is clear that plenty of progress has been made since 1992 – the year we first came to know of a planet existing beyond our solar system. For now, satisfaction can be taken in knowing that scientists are planning to keep pushing through with exoplanet research and investment.

Sagan once said, “Life looks for life.” Perhaps, in the face of the unknown, the best we can do is to continue searching in the hope that somewhere, something is also curiously looking for life.