THE HUNT FOR EARTH’S SISTER
A search that could make us rethink our place in the Universe
CHRISTMAS, 1968. THREE MEN WERE IN ORBIT AROUND THE MOON, AS FAR AWAY FROM EARTH AS ANYONE HAD EVER GONE. THEY THOUGHT THE MAGNIFICENT DESOLATION OF THE LUNAR LANDSCAPE WAS THE MOST STUNNING THING THEY WERE
GOING TO SEE DURING THEIR MISSION. BUT THEY WERE WRONG. On their fourth orbit, astronaut Bill Anders saw Earth rise above the Moon’s horizon. He photographed the moment and in doing so gave the world one of the most iconic images of the space age. This picture of the seemingly fragile blue planet Earth subsequently became a symbol to represent both the environmental movement and the wonders of space exploration. Now, a group of privately funded astronomers and engineers want to recreate that act by taking a new picture of another blue planet – one around another star.
BLUE SKY THINKING
Dubbed Project Blue, the mission aims to build and launch a space telescope with a single goal in mind: to image any planets in the habitable zones of the nearest Sun-like stars. If such planets were Earth-sized with oceans and atmospheres, then they could even “see Blue”, Project Blue’s term for finding a potentially habitable planet.
The mission is the brainchild of the BoldlyGo Institute. This not-for-profit organisation was founded by Dr Jon Morse, a former NASA scientist and White House science advisor, to investigate highly compelling scientific questions using private money from donors and crowdfunding initiatives. And there are few questions more compelling than whether there are other Earth-like planets around other stars.
The quest to find Earth-analogues, as these planets are known, began in earnest in 1995 when a pair of Swiss astronomers discovered 51 Pegasi b – the first extrasolar planet (or exoplanet) to be found around a Sun-like star. It was the size of Jupiter and not at all Earth-like, but it proved that planets were now in reach of our technological abilities.
In the decades since, almost 4,000 other exoplanets have been detected but hardly any have had their picture taken. The trouble is that planets do not generate any of their own light and instead simply reflect their star’s light. This makes them more than a billion times fainter than their parent star. Telescopes to date have been able to catch a few glimpses of large planets, but planets the size of Earth have remained impossible to image. Instead, astronomers have used indirect observations to infer the existence of the exoplanets.
Most of the exoplanets found so far were detected
HEADLINES AROUND THE
WORLD, TO DATE WE
HAVE NOT FOUND A
using NASA’s Kepler Space Telescope. Kepler tracked a star’s brightness, looking for the dip caused when a planet crossed in front of it. Its instruments were precise enough that it could see smaller rocky (also called terrestrial) planets but none of them have proved to be a twin to Earth. Although some grabbed headlines around the world for being similar, to date we have not found a true Earth-twin in the sense that it is an Earth-sized world in an orbit the size of Earth’s around a Sun-like star.
As luck would have it though, the nearest star system to the Sun contains two stars that could be extremely rewarding places to look.
Alpha Centauri is made up of three stars in mutual orbit around each other. One of these, known as Proxima Centauri, is a red dwarf star and therefore considerably smaller and cooler than the Sun. Of the others, Alpha Centauri B is similar to the Sun and Alpha Centauri A is virtually identical. These are the stars that Project Blue will target.
Existing studies of these two stars show that large planets like Jupiter are not present. So if there are planets there, all they can be are Earth-sized worlds in Earth-like orbits. “That’s what we are going to test,” says Morse.
The project proposes a small space telescope. Its mirror is just 0.5 metres in diameter, which makes it about half the size of Kepler. Yet this should be big enough for Project Blue to take direct pictures of any planets as they move around their star because the spacecraft will use an instrument called a coronagraph. It will block out the light from the central star, allowing the much fainter planet to be seen.
Don’t expect anything spectacular, though. Any planet will appear as nothing more than a single pixel of light, similar to the image of Earth captured in 1990 by the Voyager 1 spacecraft from a distance of four billion miles. Despite its lack of aesthetic beauty, it would allow scientists access to unprecedented information about the planet.
“Monitoring the brightness and the colour of planets over time is what allows you to make maps of the surface,” says Dr Margaret Turnbull of the Carl Sagan Center for Research at The SETI Institute, California, and a member of the Project Blue team. “Are there oceans? Are there continents? Are there cloud patterns? Weather patterns? Seasons? If there are, all of those things should be reflected – literally – in the colour data and in the brightness of the planet over time.”
Earth, for example, looks bluer when we are looking at an ocean than at a continent, and it is brighter when we are looking at Antarctica.
The simplicity with which the mission can be stated belies the technical challenge. No one has yet flown a coronagraph designed for taking pictures of Earth-like planets. Project Blue is working closely with NASA, which is planning a much larger mission called WFIRST (the Wide-Field Infrared Survey Telescope). It is designed to have the sensitivity of the Hubble Space Telescope but with a field of view 100 times the size and will include a coronagraph that Turnbull has been working on. Project Blue will use a lot of the ideas and technology being developed for WFIRST to provide an orbital test of how to use such an instrument to detect planets. This is why it is so important to devote the whole mission to exploring just one star system.
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EARTHRISETaken on Christmas Eve by the crew of Apollo 8, Earthrise was the first time a human had seen Earth rise from behind the horizon of the Moon. A subsequent mission,Apollo 10, took a video of the Earthrise.
ABOVE:Project Blue (left) will be launched into low-Earth orbit and will photograph planets in the Alpha Centauri system. It will use a lot of technology being developed for NASA’s much larger WFIRST mission (right)