From Gallifrey to Tatooine, planets with multiple suns feature widely in science fiction, but there are currently only ten real ‘circumbinary’ planets identified by space scientists. Dr David Armstrong from Warwick’s Astrophysics research group considers what we know about planets with two stars – and asks if life could exist there.
Planets with multiple suns in the sky have been an enduring feature of science fiction and popular culture for over a century, catching our imaginations in novels and film. Isaac Asimov’s short story ‘Nightfall’, and the planet Gallifrey in Doctor Who are two examples. Perhaps the most widely known is the planet Tattoine in Star Wars, baking famously under the heat of two stars.
It took much longer for the first of these types of planets to be discovered for real. The fascinating Kepler-16b was found in 2011, a Saturn-sized planet with two very different stars in its sky. One is similar to our Sun, if a bit smaller, but the other is a tiny, cool star – like a brighter, glowing red moon in the sky.
When we see pairs of stars orbiting each other we call them binary stars, and if a planet orbits both of those stars at once it’s called a circumbinary planet. Circumbinaries are rare, we only know of around ten, despite having found thousands of planets with only one star. But this doesn’t mean there aren’t more out there, as they are much harder to spot.
We typically find them by looking for small dips in the brightness of the stars, called transits, as the planets passes between us and the star. A bigger dip means a bigger planet. Two stars means these planets can have very odd orbits, a long way from the reliable circles of most of the planets in our Solar system. The stars fight to influence the planets, and the chaotic orbits that result from this means the transits shift around, making them harder to see.
A strange world
A circumbinary planet has some truly strange properties. The length of day and night will change, and the light from each star might be very different, even a different colour. The planet’s temperature can change as the stars move closer and further away, leading to blistering summers and frigid winters. It’s not just the temperature, the length of these seasons can be very different too, even changing week by week. It is even possible to recreate the changing seasons of Westeros with planets in a binary star system, although the setup is very specific.
We still don’t know what these temperature swings would do to a planet’s atmosphere, but one way or another life would be different on such a world. Some, maybe even most, circumbinary planets would have very small swings and therefore be as potentially habitable as any other planet, perhaps even more so if there is a wider range of energy coming in to feed any lifeforms.
We know of plenty of adaptable lifeforms capable of dealing with a huge range of temperatures, from over 100°C in hydrothermal vents at the bottom of the ocean down to sub-zero water in Antarctic sea-ice. Adapting to big swings in temperature is harder, but sheltering underground, even a small depth, would help a lot. In the most extreme cases some form of hibernation could evolve to wait out the difficult temperatures. Some lifeforms on Earth, like Tardigrades, can go dormant, even surviving the vacuum of space using a suite of clever tricks.
But even with their survival tricks, tardigrades still need to return to a more typical range of temperatures in order to revive. So if you could guarantee the winter would come to an end, perhaps there is a niche life could fill. Maybe even a cantina. We are still yet to find out.
10 September 2020
The Astronomy and Astrophysics group at Warwick is interested in a huge range of scales across the Universe: planetary systems, how they form, live and die; stars, stellar binaries and and the exotic physical processes that they allow us to explore; as well as the transient events which mark the end of stellar lifetimes and the galaxies stars inhabit across the Universe. The group started in September 2003 and is both an observational and theoretical group. The group makes use of a wide range of ground-based telescopes, such as ESO's Very Large Telescope (VLT) in Chile and the Isaac Newton Group of telescopes (ING) in the Canary Islands, or the Atacama Large Millimetre Array (ALMA), as well as space telescopes such as NASA's Chandra and ESA's XMM-Newton X-ray observatories and the Hubble Space Telescope. The Warwick astro group partners in the four large spectroscopic surveys (DESI, SDSS-V, WEAVE, and 4MOST) that will start operations throughout 2020-2021.
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