What are exoplanets and can they sustain life?
‘It seems possible that any planet with a mass a few times that of Earth could have an atmosphere similar to the Earth’s’
Artist’s impression of the planetary system around HD 69830. Image: ESO
An artist’s concept allows us to imagine what it would be like to stand on the surface of the exoplanet TRAPPIST-1f, located in the TRAPPIST-1 system in the constellation Aquarius. Illustration: Nasa/Getty Images
Since the late 20th century nearly 3,000 planets have been discovered outside our own solar system, revolutionising the way we think about the universe, and opening up our minds to ideas such as the possibility of life elsewhere.
These so-called exoplanets, which orbit their own stars, are now expected to exist in abundance throughout the Milky Way, and research is under way into how some of these might be able to support life.
It is thought that a number of factors need to be just right for initiating and maintaining the evolution of life on other planets. These factors include the planet’s temperature, which should be neither too hot nor too cold (and should not vary too dramatically), and – crucially – the availability of liquid water. But our understanding of these constraints is only in its infancy.
“Regarding life, the current – and very simplistic – view is that the planet has to have liquid water at its surface,” says Prof Aline Vidotto, at the school of physics, Trinity College Dublin.
“This is mainly determined by the temperature of the planet, which is set by how much irradiation it receives from the star which it orbits,” she says.
A planet’s temperature therefore depends largely on how far it is from its star, and the region around a star where just enough radiation is received for liquid water to exist is called the habitable, or Goldilocks, zone.
Discovering new atmospheres
An important factor in the assessment of whether a planet could harbour life is the make-up of its atmosphere. Exoplanets come in a range of sizes and compositions, from rocky Earth-sized objects to larger ice and gas giants such as Jupiter. The atmospheres of these planets depend entirely on the nature of the planet itself: its size, chemical composition, and distance from its sun.
According to Prof Andrew Shearer, at the Centre for Astronomy, NUI Galway, it seems possible that any planet with a mass a few times that of Earth could have an atmosphere similar to the Earth’s. However, “in the case where the planet is much heavier, the atmosphere will be dominated by hydrogen, and unlikely to harbour life”, he says.
Current technology allows astronomers to observe and assess the various kinds of atmospheres on exoplanets. Astronomers use spectroscopy – the study of the interaction between matter and electromagnetic radiation (including visible light) – to determine the atmospheric properties of an exoplanet.
“By careful spectroscopy, the light from the planet can be distinguished from the light from the parent star (the star which it orbits),” says Prof Shearer.
“The problem is that the planet will be much much fainter than the parent star,” he says, “meaning we need careful techniques to separate the planetary and stellar light.”
Using these methods scientists have been able to detect a range of atmospheric properties on exoplanets, including pressure-temperature profiles, chemical compositions and energy circulation patterns. Chemical composition is particularly important, because if the atmosphere contains significant greenhouse gases, these can amplify the planet’s temperature beyond that which would be typical for its distance to the sun.
Weather on exoplanets
The weather on exoplanets is another factor which could affect a planet’s chances for sustaining life. Recently astronomers have been able to observe the weather on the exoplanet HAT-P-7b, with clouds potentially made of corundum, the same mineral that sapphires and rubies are made from on Earth.
According to Prof Vidotto you can detect weather by performing transmission spectroscopy at several wavelengths.
“If the planet is covered in thick clouds then all the spectroscopic features of the atmosphere are no longer seen,” she says.
The existence of clouds is important when considering potential opportunities for supporting life. Too many clouds, for example, would reflect sunlight, thereby cooling a planet to temperatures which are not considered suitable.
Prof Shearer admits that we don’t yet know a lot about weather patterns on exoplanets. Referring to HAT-P-7b, he says “There is possible evidence of clouds on this Jupiter sized exoplanet, but the high temperatures make the place extremely hot, burning at over 2,000 degrees.”
In the future, new technologies are set to generate a broader understanding of exoplanetary atmospheres, and shed further light on the potential for life on these worlds. With regards to exoplanetary weather, Prof Shearer concludes that not far from now “satellites such as Nasa’s Plato or the E-ELT (European Extremely Large Telescope) should be able to make weather forecasts for certain exoplanets”.
THE SUN’S ATMOSPHERE
In addition to planets, and some moons and comets, stars are also host to atmospheres. In our solar system, our host star – the sun – boasts an atmosphere consisting of several layers, primarily the photosphere, chromosphere, and the corona.
The photosphere is the lowest layer of the solar atmosphere, extending upwards to a height of around 500km. It is here where the well recognisable dark sunspots appear, caused by the sun’s magnetic field as it breaks through the surface. The photosphere is also the source of solar flares which burst out for hundreds of thousands of kilometres above the surface.
Above the photosphere is a region approximately 2,000km-thick known as the chromosphere. With temperatures ranging up to 20,000 degrees the chromosphere is much hotter than the photosphere below it (which burns at around 3,800 degrees).
Beyond that is is the corona, hotter again in places reaching 20 million degrees. It can only be seen during a total solar eclipse as plumes of ionised gas are ejected outwards into space. It is these gases which travel through the solar system in what is known as the solar wind, and this has a large control on the behaviour of the Earth’s magnetic field.