Habitable exoplanets

Jack Schofield describes the newest development in the quest to find habitable exoplanets

Douglas Adams’s Hitchhiker’s Guide to the Galaxy begins with the observation that “space is big. Really big. You just won’t believe how vastly, hugely, mind-bogglingly big it is.” The vast scale of the cosmos and its billions of galaxies and stars may lead us to think of the presence of life as inevitable, as if it is bound to occur somewhere. Despite our extensive searches of the skies, however, we have yet to find any definitive signs of life on any celestial bodies. Life on Earth is all that we know of to date. The field of astrobiology, which combines the study of the life, geology and atmospheres found on Earth with analysis of distant planets, hopes to change this.


While we have been thinking and talking about alien life for many years, astrobiology research has only really been treated with real scientific rigour in recent decades. In 1995, two astronomers, Michel Mayor and his student Didier Queloz, identified 51 Pegasi b, a small planet located 50 light years from Earth. This was the first time that a planet had been found in orbit around a star other than our Sun. Since this early work, many more of these extrasolar planets, or “exoplanets”, have been found, with over 3,000 confirmed planets, 352 of which are terrestrial or rocky. These planets, small, rocky bodies that emit no light of their own, are difficult to spot in the vastness of space. They are, however, some of the most interesting objects hidden in the cosmos. If extraterrestrial life is anything like what we are familiar with on Earth, these planets are our best hope for finding it.


In December 2015, a collaboration led by Michaël Gillon from the University of Liège in Belgium studied an “ultra-cool” dwarf star roughly 40 light years from Earth which they called TRAPPIST-1. Using a telescope based in the mountains near Chile’s Atacama desert, they discovered three Earth-sized planets around the star. Given the imaginative names TRAPPIST-1b, -1c and -1d, these planets were shown to orbit quite close to their star, with years much shorter than those here on Earth. In February 2017, NASA announced that four more planets had been found in the TRAPPIST-1 system. This leaves us with a total of seven planets, the most we have ever found orbiting a single star. Why should we get excited about some rocks whirling about a dim, distant star? This is not stamp-collecting by braggadocious astronomers finding the biggest and most complicated star systems. These planets may be more promising than any found to date. Based on estimates of their mass, all seven of the planets are Earth-like (rocky), while the low temperature of the star suggests that liquid water could be found on their surfaces. As well as this, three of these planets are found in the habitable zone of the star, positions which can, in theory, support life.


Despite the fanfare and excitement over this discovery, the ability of these new habitable planets to support biological life is far from certain. Any theories that we concoct about life on these or any other exoplanets are speculative and based on a number of assumptions.    


While the discovery of these planets is fantastic, it is worth asking what scientists means when they call a planet “habitable”. In the field of astrobiology, the presence of liquid water is the essential criterion when classifying a planet as habitable. Habitable does not mean that we have seen oceans teeming with life on these planets, it just means that the planets have the right mass and distance from their host star to possibly contain water. Astrobiologists even divide habitable planets into different classes based on factors such as the rate at which they lose their water over time or whether the water is located below the surface. Exoplanets are judged habitable by using simulations based on their distances from their host planets, not experimental observations of real water bodies.


We make our assumptions about a planet’s ability to sustain water based on its distance from its host star, the strength of the planet’s magnetic fields and the size/temperature of the star. Water can exist in its liquid, solid or gaseous state depending on its temperature and pressure. If an exoplanet is positioned close to a host star, in the inner edge of the habitable zone, its water can be completely lost as water vapour or become part of the upper atmosphere. The other extreme is the habitable zone’s outer edge, beyond which water is completely frozen and inhospitable. Calculating these ideal distances, however, only makes sense if we know the size and energy of the star, as stars have different habitable zones depending on their temperatures. Liquid water will be more probable on a planet close to a hot star than a cooler one. The habitable planets around TRAPPIST-1 are quite close to their host, but as it is an ultra-cool star, water may be sustained.  


This assumption is only valid if the planet has an atmosphere which can retain its water. There may be, as findings such as the TRAPPIST study suggest, many rocky planets orbiting at the right distances from their stars. However, the kind of atmosphere that surrounds Earth could be more unique. An atmosphere is composed of compounds in their gaseous states. If a planet is to hold on to these gases, its gravity must be strong enough to prevent them from escaping into space. When studying the tiny gas molecules on exoplanets which are light years away, scientists use a technique called spectroscopy. Spectroscopy compares the light that comes from a star normally with the light which passes through an exoplanet when the planet makes its orbit. Based on the differences between these light profiles, we can detect the presence of particular molecules. One of the first uses of this technique demonstrated the presence of sodium in the atmosphere of planets around a star which is around 150 million light years from Earth.


Moving forward in the study of exoplanet atmospheres, we can do more than identify potentially habitable planets. We may also be to detect indirect evidence of life in exoplanet atmospheres: biosignature gases which are produced by living things. Exoplanet atmospheric studies have traditionally searched for biosignatures prevalent on Earth, such as oxygen (O2), ozone (O3), methane (CH4) or nitrous oxide (N2O), all gases involved in biological energy capture. Using these as biosignatures is problematic though, as these gases are not always definitive signs of life; they can also occur naturally on planets or be produced by geological/volcanic processes, which gives rise to false positive signs of alien life. To avoid this problem, gases which are distinctive signs of biological processes, such as sulfur compounds, volatile organic chemicals or halogenated hydrocarbons, can be studied. When examining these gases in the atmospheres of exoplanets, we could be more confident in calling them signs of life.


These criteria make it seem as if the search for life can be boiled down to a search for extraterrestrial water. The life that we are familiar with here on Earth is built with carbon molecules, using water as a solvent. Water is no ordinary solvent because its unique properties facilitate the biochemistry which is essential to life. Water can organise chaotic biomolecules into ordered structures according to their solubility, participate in chemical reactions, and stabilise molecules with hydrogen bonds. From what we have seen so far, no life is possible without liquid water. Alternative biochemistries based on solvents such as liquid ammonia or hydrocarbons such as methane have been proposed. Based on our experience on Earth, these alternatives could not produce the astounding chemical diversity that is seen in the life that we know of; this conclusion is uncertain, however, as it is based on the limited perspective of Earthlings. Life based upon water and carbon molecules is all we have ever known. Our framework of understanding for biochemistry depends on these assumptions; if life can follow rules different to those with which we are familiar, these searches may not be able to find it.
We should be excited about pioneering works such as TRAPPIST. It is, without doubt, a phenomenal discovery. If we are to eventually find life on distant exoplanets, however, we need to acknowledge our assumptions, and our biases.

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