Back in the 1940s most astronomers would have said that we must be alone in the Galaxy, if not the Universe, because the Solar System was believed to be unique. The Nebular Hypothesis for the origin of the Solar System, advanced by Kant and Laplace in the 18th century, suggested that most stars if not all would have planets. In 1930 Sir James Jeans suggested instead that the planets had formed out of a cigar-shaped filament drawn out of the Sun in a grazing encounter with another star. Stars are far enough apart that such an event might happen only once in the history of a galaxy, so we would be alone.
Coincidentally or not, Jeans published his Collision Hypothesis within two years of the first novel of interstellar travel, The Skylark of Space by E.E. (‘Doc’) Smith. It wasn’t great literature, but it filled the Galaxy with planets and intelligent life. Science fiction writers simply ignored Jeans and have strewn the Milky Way since with both, and have turned out to be right, at least about the planets.
Even in 1967, when I launched the discussion project which led to my book Man and the Stars, Dr. (later Prof.) A.E. Roy of Glasgow University felt that the topic of extrasolar planets was too controversial to put his name to.
In the mid-1950s Hoyle and von Weisäcker had countered Jeans with a new model suggesting planets would be common, and Archie was inclined to agree, but didn’t want to risk his reputation by saying so. (This was after he’d published his first thriller as ‘Archie Roy’, but before he ‘came out’ as the founder of the Scottish Society for Psychical Research.) The nearest stars to us are Alpha Centauri and its companion Proxima Centauri; Alpha is a double star 4.3 light-years away and Proxima is a red dwarf, too faint to be seen with the naked eye from here, at 4.2 light-years. Alpha Centauri A and B orbit each other at about the mean distance between the Sun and Uranus, and Archie Roy thought that they might have planets like the inner ones of our Solar System. Both stars are like our Sun, though smaller, so they could support life.
Proxima Centauri is a tenth of a light-year out from them, and back then we weren’t even certain that it’s gravitationally bound to them, though it turned out to be. European Southern Observatory observations have greatly improved accuracy for the star’s measured velocity with respect to us, making it seem much more likely that it is gravitationally bound to the Alpha Centauri system, despite the comparatively large separation, with an orbital period on the order of 500,000 years. All three stars are now thought to be about the same age – about 6 billion years, rather than 9 billion years as previously thought, so about 25% older than the Solar system. Interaction between the stars may have expelled Proxima to its current distance, raising the interesting possibility that Proxima b was an outlying planet, moved sunwards in the encounter, or possibly even stolen from one of the Alpha stars. If it formed beyond the ‘frost line’ of the protoplanetary disc, it could well have been ice-covered initially and have liquid oceans now. (Anon, ‘Orbit of Proxima Centauri Determined After 100 Years’, European Southern Observatory, Announcement 16089, 22nd December 2016.) Proxima is a red dwarf star, much dimmer than our Sun, and if it had an earth-sized planet which could support life, it would be so close to Proxima that it would orbit in only ten to twelve days.
In the 1950s science fiction took considerable liberties with the properties of the Alpha Centauri stars, especially in the comics. In the Dan Dare stories ‘The Man from Nowhere’ (Eagle, 2nd November 1955) and ‘Rogue Planet’ (17th August 1956) ‘the three suns of Los’ were shown closely grouped and although the primary and the smallest one were correctly shown as yellow and red, Alpha B (‘Mina, the Mother’) was green and surrounded by an equatorial ring, which would mean either that it was spinning up, or its surface temperature was increasing at the end of its stable life. Green stars are unusual and the classifications often depend on the perception of the observer. Even more remarkably the habitable planets Cryptos and Phantos managed to orbit all three stars, in orbits so convoluted that they made a close approach to each other only once in ten thousand years. In reality, to have stable orbits and climates they would have to orbit either Alpha A or Alpha B, and if they orbited one each, their closest approaches would come every 80 years. In 2012 it was thought that a planet had been discovered orbiting Alpha Centauri B, but it has not been confirmed and is considered suspect. The Alpha Centauri and Proxima stars are being studied intensively in Breakthrough Listen’s ‘Pale Red Dot’ project, and there are plans for a ‘Project Blue’ orbital telescope dedicated to the study. (Matt Williams, ‘Project Blue: Building a Space Telescope that Could Directly Observe Planets around Alpha Centauri’, Universe Today, 25th October 2017.)
Proxima Centauri was discovered in 1915 by Robert Innes. In The Giant Leap, Mankind Heads for the Stars, the late Adrian Berry said that due to modesty, Innes signed his papers as ‘R.I.’ and so received no credit. However I’d come across occasional references to a nearby ‘Innes’ Star’ which wasn’t listed as such in catalogues of nearby stars, and that is indeed Proxima. Adrian also cited a 1998 report that the Hubble Space Telescope’s Faint Object Camera had discovered a planet of ten Jupiter masses orbiting Proxima. In 2016 Proxima was found to have a planet, confirmed in 2020, with 1.17 times the mass of the Earth, and located in the habitable zone. It could be habitable, and I’ll come back to that in a future article, but it could be a super-earth or even a water-world, with no solid surface. Still more recently a more distant giant planet has been confirmed, with the mass of Neptune and possibly the diameter of Jupiter, but not as large as the one suspected in 1998. And on February 10th this year, just as this article was going to press, a third planet was announced, orbiting between the other two and having roughly the same mass as Mars.
The next nearest sunlike stars are Epsilon Eridani, 61 Cygni A and B, Epsilon Indi and Tau Ceti, lying between 10.8 and 12 light-years from us. Barnard’s Star, 6 light-years away, is a red dwarf and was thought in the 1960s and early 70s to have a planetary system, the target of the British Interplanetary Society’s ‘Project Daedalus’ interstellar probe study. From the 1940s it was thought that both 61 Cygni and Epsilon Eridani, two of the nearest stars like the Sun, had superjovian gas giant planets, but the apparent evidence was knocked down in the 1970s. The Infra Red Astronomical Satellite, IRAS launched in 1983, discovered that Epsilon Eridani was less than a billion years old and still surrounded by dust, out of which planets might be forming now. In 2000 a Jupiter-mass planet was discovered, and in 2008 an asteroid belt was found, separated by ‘Kirkwood gaps’ which imply the presence of a fully formed planetary system, although the outer ring corresponding to our Kuiper Belt is still evolving. Tau Ceti is now reckoned to have at least five planets, one or more in the ecosphere; it’s thought to be at least a billion years older than the Sun, yet it still has a huge retinue of comets, so many that constant bombardment of the planets may make life there impossible.
Now, however, ‘exoplanets’, as they’re now called, are known to exist in huge numbers. The process began in the 1980s when IRAS detected discs of material orbiting new, giant stars such as Fomalhaut and Beta Pictoris. Ongoing studies found distortions in such discs, apparently due to the gravitational pulls of protoplanets, and heating, apparently due to collision events like the one which formed the Earth-Moon pair. The first actual planets were found orbiting a neutron star, and present discovery techniques still favour discoveries around low-mass stars like red and brown dwarfs, but planets have now been found orbiting stars of all types – even double stars (like Tatooine in Star Wars), which many thought to be impossible. Planets are being found even by ground-based telescopes, but huge numbers are being found in the data from NASA’s Kepler survey of stars in Cygnus, even though the satellite is now retired, and meanwhile TESS (Transiting Exoplanet Survey Satellite) is racking up new discoveries all the time.
At first the planets found were all gas giants, and ‘hot Jupiters’ and ‘hot Neptunes’ are so common that people ask why our giant planets are so far out, lucky though that is for us. But as techniques improved it became possible to detect ever smaller ones, starting with ‘hot Venuses’ and ‘hot Earths’ – some of them so hot that they have atmosphere of gaseous rock and metal. For example 55 Cancri e is so hot that it must be a ‘lava world’, unless its surface is composed of diamond.
Planets smaller than Earth were detected in 2011, and a family of them was discovered orbiting Kepler-20, a type G star like the Sun. Earth-sized worlds in the habitable zones around sun-like stars are now within the detection range, even though NASA’s Terrestrial Planet Finder was cancelled in 2007. Many of the worlds found are ‘mini-Neptunes’, by mass, but that doesn’t necessarily mean extreme surface gravity, because that’s also determined by the density of the planet, particularly the crust. If the density is too low, however, we could be looking at ‘water worlds’, with water comprising up to half of their mass and oceans up to 100 miles deep, at which point the pressure causes water to take up strange forms unknown on Earth. Stephen Baxter’s new novel Galaxias postulates that a single such life-form managed to colonise the Milky Way 10 billion years ago, and has had things all its own way since, suppressing the inhabitants of worlds like ours in case wreck the neighbourhood within runaway technology.
Some of the gas giants discovered could have habitable, Earth-sized moons, including one of the very first, orbiting 47 Ursae Majoris, and the imaginary planet of Avatar. Early in 2012, again, an example was found orbiting a double star, Kepler-16; and a giant planet with rings was found orbiting a brown dwarf. The occultation pattern as it passed in front of its star resembled the one when the rings of Uranus were discovered in 1977, and it indicates the presence of moons, plural, whose gravitational pulls generate the gaps. If we don’t find an Earth-sized world on its own, we may find one orbiting a gas giant (like Endor in Star Wars) any time now.
In 1962, Harlow Shapley suggested that there might be stray planets wandering in interstellar space Matematica y Fisica Serie A, 14, 69-75, 1962) . At least a dozen are now known within the Milky Way, and there are indications of thousands in other galaxies. Poul Anderson wrote a novel in 1968 called Satan’s World, in which he imagined how dramatic events might be if one passed close to a star. But Shapley’s title. ‘Crusted Stars and Self-heating Planets’, implied even more intriguing possibilities. He was thinking about internal heating by radioactive decay, but gas giant planets and brown dwarf stars, all too small to run nuclear reactions, are heated by internal contraction, and brown dwarfs have weather patterns like gas giants, so the possibility of life in stray planets can’t be ruled out. And there may be more of them than there are stars in the Milky Way (Paul Scott Anderson, ‘There May Be 50 Billion Free-Floating Planets in Our Galaxy’, Universe Today, March 10th 2019.) So Galaxias had better watch out.
Some of the biggest surprises have come from the red dwarf stars, the smallest which can sustain themselves by nuclear fusion. Their numbers are much greater than was supposed, perhaps 200 billion of them in a Galaxy of 400 billion stars, more than twice as many as there were thought to be in the 1960s. So many red dwarfs have been found to have planets that it’s now thought they may all do, and that may apply to orange dwarfs and yellow ones like the Sun as well. It used to be thought that they couldn’t have many planets, because orbital resonances would cause them to collide and be expelled. But Kepler-186, Gliese-832 and Kepler-1649 are all orbiting red dwarfs with families of planets, and doing very nicely. Then the Transiting Planets and Planetesimals Small Telescope in Chile found three Earth-sized planets skirting the habitable zone of a red dwarf designated TRAPPIST-1, 40 light-years away. Their orbital plane allowed multiple transits of the star, seen from here, and follow-up infrared observations allowed NASA’s orbiting Spitzer telescope to detect four more Earth-sized planets by February 2017, at least three of them in the habitable zone, with volcanic activity possibly making the outer ones habitable as well – and volcanic activity was likely, due to tidal effects, because they were so closely grouped that they would be seen from one another as discs, large enough to show continents and cloud features. Tidal effects would also stop them from taking up trapped rotation with respect to the star, just as Carl Sagan and I.S. Shklovskii had suggested might happen in such a case. (Intelligent Life in the Universe, Holden Day, 1966. NASA’s artists lost no time in adding a TRAPPIST-1 poster to their series of retro travel posters, featuring objects in the Solar System and exoplanets.
Observations from the Konkoly Observatory in Budapest seemed to rule out habitability for any of the new planets. TRAPPIST-1 is an exceptionally active flare star, with events hundreds of thousands of times more powerful than the Carrington Event of 1859, the most powerful solar flare on record. At that intensity a planet’s atmosphere might take 30,000 years to recover, and as they happen every 48 hours, they would strip the TRAPPIST-1 planets of air and water. (Evan Gough, ‘TRAPPIST-1 Is Showing a Bit Too Much Flare’, Universe Today, April 4th, 2017). But no sooner had that conclusion been published that the Hubble Space Telescope discovered hydrogen and oxygen in the ecosphere of the star, showing that the planets there were indeed water-bearing and might even be water-worlds. (Matt Williams, ‘Hubble Spots First Indications of Water on TRAPPIST-1’s Planets’, Universe Today, 31st August 2017.)
The flares of TRAPPIST-1 are not particularly good news, given that the star is thought to be about a billion years older than the Sun. Still, because of the greater mixing that should occur within a relatively small star, it’s thought that in the longer term red dwarf stars will be stable – and if so, as Arthur C. Clarke pointed out in Profiles of the Future (Gollanz, 1962, updated 1999), their stable lives will run into trillions of years. The entire lifetimes of all the other stars will only be flickers at the dawn of red dwarf history. If they all have planets, and they evolve intelligent life (or it travels to them), what they might accomplish in that time is wholly beyond our imagination. And yet, as Arthur points out, they may envy us – because we knew the Universe when it was young.
When we do find a life-bearing world, the next challenge will be to image it. By 2013, already images had been obtained of planets orbiting Fomalhaut and HR 8799 – the Fomalhaut one turned out to be only a dust cloud, but the four planets of HR 8799 are real. Once we can see an Earthlike planet, the next step is to obtain a spectrum of its atmosphere. If there’s water vapour, there could be ‘life as we know it’ (sun glint could show the presence of oceans); free oxygen would almost certainly indicate plant life; methane with free oxygen could be due to volcanic activity, but could indicate animal life. Study of earthlight, reflected on to the Moon, was found to show complex organic compounds in Earth’s atmosphere, and the same technique could clearly indicate the presence of life on an exoplanet. Water, sand, leaves and forests all show very different biosignatures. Amazingly, to someone of my age, serious papers are now being published on whether even cities could be detected at interstellar distances.