5x5 Wonder Wednesday: Five incredible exoplanets
The first planet outside our Solar System was discovered in 1998, although it wasn’t confirmed until after others had been found definitively. To date there are 3,501 confirmed planets in 2,623 planetary systems, including 592 multiple planetary systems, with thousands more candidates waiting to be confirmed. This month saw the confirmation of a planet orbiting Earth’s nearest stellar neighbour, Proxima Centauri, so here are five of the most incredible exoplanets found so far.
1 The first confirmations: PSR B1257+12 b & c The discovery of pulsar PSR B1257+12 in 1990 was followed two years later by the revelation that irregular beats in the radio waves sent out by the star were being caused by two small planets, with a third identified in 1994. They’ve since been called Draugr, Poltergeist and Phobetor, with the parent star named Lich — all undead mythological creatures. They’re 2,300 light years away.
A pulsar is a neutron star, which is incredibly dense and spins very fast, emitting beams of radiation, including radio waves, from its poles. If you’re in the line of these beams they beat regularly and were first mistaken for alien radio signals. The reliability of these pulses makes it easy to detect planets around them causing wobbles, but it’s also very uncommon, because neutron stars are usually the leftover from the supernova explosion of a giant star. Only one other pulsar system with a planet has been confirmed.
PSR B1257+12 was formed unusually, by the merger of two white dwarf stars, most likely a binary system of two stars similar to our own sun which reached the end of their lives in a less dramatic fashion. The innermost planet, Draugr, is still the smallest exoplanet known, with a mass less than double that of our Moon, but a neutron star would shower its planets in deadly radiation, especially at the very close distances where these exist.
2The Hot Jupiters: 51 Pegasi b It’s 21 years since astronomers first confirmed the discovery of a planet outside our solar system, which was officially named Dimidium in December last year. It whizzes about its sun — 51 Pegasi—in just over four days, much closer than our innermost planet, Mercury.
51 Pegasi is a stable star that’s much like our own Sun, which made it ideal for the radial velocity method used by Michel Mayor and Didier Queloz of the University of Geneva to detect Dinidium by the slight changes its orbit causes to the spectrum of light given out by the star.
Planets like this are called Hot Jupiters because they’re almost certainly made of gas, and many more have been found since. They’re tidally-locked to their Sun, presenting the same side to it and inflated to vast size by the heat it receives — it has half the mass of Jupiter but is 50% larger, with a temperature of about 1000°C.
2 The rocky worlds: CoRoT-7b The first Earth-like exoplanet was confirmed in 2009, and it wasn’t that Earth-like. True, it’s about one-and-a-half times the diameter of Earth, but it weighs about five times as much and orbits its parent star in about 20.5 hours, so close that its surface temperature between 1800°C and 2600°C. It would have a lava ocean on the tidally-locked side facing the sun, but it could have a wispy atmosphere of evaporated rock.
Many of the rocky worlds discovered so far have been like this, because their short orbits make it possible to detect using the transit method, where astronomers measure changes in the brightness of the parent star as a planet passes in front of it.
Many of the other rocky planets discovered are called Super Earths because they’re either up to 10 times heavier or up to twice as wide as the Earth. There’s been a huge rise in the discovery of Super-Earths and Earthlike planets since the launch of the Kepler space observatory in 2009, which has discovered more than 1,200 planets in 440 star systems, with more than 3,000 candidates to be confirmed.
3 The cold wanderers: OTS 44 Not all planets beyond our solar system bask in the warmth of a star they can call home. These rogue planets are smaller than brown dwarfs (stars which are just too small to ignite nuclear fusion in their core), and are often moving very fast, suggesting that they were ejected from the solar system where they formed.
OTS-44, found in 1998, is around 11.5 times the mass of Jupiter and some 554 light years away from Earth. It is believed to have a disc of particles and ice around 10 times the mass of the Earth which could form its own moons, but many rogue planets are much smaller. Cha 110913–773444 is eight times the size of Jupiter, but the recently-discovered CFBDSIR 2149–0403 could weigh as little as four times Jupiter’s mass. Astronomers are confident they will find much smaller bodies wandering in the depths of interstellar space, and the larger rogues are very likely to have moons of their own.
Water and methane have been detected in the atmospheres of at least two rogue planets, and it’s been calculated that without a star to blow away their atmospheres, geothermal energy could keep them warm enough to melt ice, while a magnetic field would protect them from cosmic radiation, although there would be no light except the distant stars.
4 The Goldilocks planets: Kepler-452b There’s a region around every star where the surface temperature of a planet would be warm enough to melt ice but cool enough to stop water turning to steam. This is the Goldilocks zone, and given that liquid water is the essential ingredient for all life on our only sample planet, the Earth, it’s the best place to look for planets which might host other life forms. It’s also known by an unsexy scientific name: the circumstellar habitable zone, or CHZ.
Astronomers began finding planets in the habitable zones of stars as early as 1996, but many were either gas giants or tidally-locked rocky planets which orbited small stars in tight orbits. The gas giants could have Earth-sized moons, but there’s no way to detect these as yet. It was 2015 before NASA’s Kepler space telescope found a rocky planet in the right spot around a Sun-like star, with a year of 385 days. It’s 60 per cent larger than Earth, but crucially it’s not tidally-locked so it would have days and nights to regulate its temperature.
Kepler-452b is one of over a dozen planets so far detected by the Kepler mission which are roughly the right size and distance around stars which are expected to be stable, and many orbit alongside other planets, just like Earth. All have been found by indirect methods like the wobbles and dips in light of their parent stars — the next step is telescopes which can take direct images of their dim light, reflected in the glare of their home star. There may be around 40 billion in our Milky Way galaxy alone.
5 The one next door: Proxima Centauri b With exoplanets so common, it might not have been a surprise to find one orbiting our closest neighbour, Proxima Centauri b. It was a surprise that it’s roughly Earth-sized and in the habitable zone. Unfortunately, Proxima Centauri is a dim red dwarf star, with just 12 per cent of the mass of our sun, and PCb is 25 times further in, with an 11-day orbit that’s — you guessed it — tidally locked. Proxima Centauri is also a violent star, prone to solar flares which could scorch the planet of anything lifey.
On the other hand, there might be another planet orbiting further out with a 200-day year, but that would be much cooler as well as safer. It might also have a complex orbital relationship with the nearby Alpha Centauri binary star system. We just don’t know yet. It’s certain that the James Webb Space Telescope will be turned towards our neighbour when it launches in 2018, and astronomers will be working on new methods for taking measurements with existing telescopes while dreaming up another generation of instruments.
As for visiting, even at 4.2 light years it’s impossibly distant with today’s chemical rockets, and I can’t imagine Donald Trump getting excited about other planets, unless he could build a golf course or casino there, or convince America that it was about to be invaded by little green men. And the trouble with space is that Putin can’t take his shirt off up there. But if the world comes to its senses (they’re far from the only insular idiots on the global stage) and decides to do something exciting, Proxima Centauri b might spur our imaginations to send probes beyond the solar system in human lifetimes, and I imagine that might bring a few spin-offs for improving life closer to home.