In 1990, NASA’s Voyager 1 spacecraft briefly looked back from its journey out of the solar system, capturing a view of the faraway Earth. Carl Sagan called it the “pale blue dot.” From more than 6 billion kilometers away, beyond the orbit of Pluto, it seemed remarkable that our planet was even visible. But the most remarkable thing about the image was what one could learn about the Earth, even from so far away. Lingering over that pale blue dot, measuring its fluctuating brightness and color, a clever observer could discern that our planet had clouds, oceans, continents, and perhaps even a living, breathing biosphere.
As small and faint as the Earth is in that iconic image, if it were observed across the much greater distances of interstellar space it would be far smaller and fainter still, and almost lost in the ten-billion-times-brighter glare of the sun – a bit like a firefly fluttering next to a gigantic searchlight. Astronomers have found nearly all of the thousands of exoplanets now known through more indirect means, watching for stars that periodically wobble or dim from unseen retinues of worlds.
To actually distinguish those planetary fireflies from their stellar searchlights, to glimpse all the pale blue dots that might exist in our galactic neighborhood, you probably need a very large, prohibitively expensive observatory in space. However, if you're willing to gamble, and only look for alien Earths around only a carefully selected sample of nearby stars, you might get lucky with a much smaller and more affordable space telescope.
Order-of-magnitude estimates suggest a thousand stars would be within reach of a telescope with an 8-meter mirror; a 4-meter might let you look at about a hundred. Both are still firmly in the realm of multibillion-dollar projects. But what about a telescope custom-built just to image Earth-like planets that might exist in not-too-hot, not-too-cold habitable orbits in the Sun’s very closest neighboring star system, the binary stars Alpha Centauri A and Alpha Centauri B? How big would that space telescope have to be, and how much might it cost? The answer might surprise you.
According to Ruslan Belikov and Eduardo Bendek, two research scientists at NASA’s Ames Research Center in California, a 45-kilogram space telescope with a 30-to-45-centimeter mirror would be sufficient to deliver images of rocky planets in the habitable zones of either Alpha Centauri A or B. That’s smaller than some of the telescopes you can buy on Amazon.com, though you can’t purchase a planet-imaging space observatory off-the-shelf quite yet. Belikov, Bendek, and their collaborators call the concept ACESat – the Alpha Centauri Exoplanet Satellite – and have submitted it to NASA in response to the agency’s October 2014 call for proposals for Small Explorer missions, which have budgets capped at $175 million. If selected, the mission would be ready to launch no later than the end of 2020.
As the pair explained in back-to-back public presentations at a recent meeting of the American Astronomical Society in Seattle, Washington, ACESat’s small size and relatively low cost is possible in large part due to a cosmic accident. Other than our sun itself, Alpha Centauri A and B are by far the brightest sun-like stars in Earth's sky, thanks to being atypically close to us – only a relative stone's throw away, at some 4.4 light-years distant. The brightness of both stars also means any planets they harbor should be relatively bright, too. According to Belikov, a mirror Earth in the Alpha Centauri system would be about a hundred times brighter in reflected light than the “typical case” for a pale blue dot around a nearby star. That boost in brightness would make worlds in Alpha Centauri’s habitable zones much easier to image, provided that first the contaminating starlight can be somehow accounted for and removed.
ACESat would carry a device called a phase-induced amplitude apodization coronagraph, an instrument designed to blot out starlight with high efficiency so that the far fainter light of accompanying planets can be seen. All coronagraphs, however, tend to leak small amounts of stray starlight into a telescope, and most are designed only to null the light of one star at a time, not of two, such as would be required for the Alpha Centauri system. ACESat would also carry a smaller, computer-controlled deformable mirror to correct for leaks on-the-fly, rapidly altering the mirror's shape to deflect the intermingled light of both stars away from its planet-seeking detectors. Even then, stray stellar photons would still leak through in sufficient numbers to potentially obscure or even masquerade as planets. ACESat’s solution to this last challenge is essentially to stare uninterrupted at both stars during a two-year mission, taking 20,000 20-minute exposures that can then be combined and scrubbed of photonic noise with image-processing software.
“This is the kind of high-risk/high-payoff mission that needs to be done,” says Olivier Guyon, an astrophysicist at the University of Arizona in Tucson who invented the variety of coronagraph ACESat would use. “If Alpha Centauri has no habitable planets, then the science return here is very small, but in this case it makes sense to try, just because the system is so much easier to look at than other stars. There is a chance that this will be a game-changer, and we’d also get to test technologies we need anyway for future, larger missions.”
In his presentation, Belikov offered a demonstration of ACESat’s image-processing techniques, showing the colored dots of a virtual Venus, Earth, and Mars slowly coalescing around Alpha Centauri B from thousands of stacked frames of shimmering, simulated starlight. Each planet’s changing brightness across five different wavelengths in ACESat’s observations would help reveal basic details, such as whether each world has an atmosphere, or even liquid water at its surface. Even so, ACESat’s modest capabilities would be unable to detect obvious signs of life, such as the oxygen from photosynthetic plants that fills Earth’s air. “The point is that this is a pathfinder to see if there are planets around Alpha Centauri,” Belikov said. “And then a larger mission could easily do follow-up spectroscopy” to look for life.
Some scientists already believe our nearest neighboring star system has planets. For decades, theorists suspected that planets would only form with great difficulty in binary-star systems, but observers have proved them wrong in recent years by finding example after example of worlds with twin suns. Orbital dynamics still place limits on what can exist around Alpha Centauri’s stars – planetary orbits further out than about 2.5 times the Earth-Sun distance would be unstable around either star, and earlier surveys have ruled out the presence of any close-in gas-giant planets. In 2012, one planet-hunting team monitoring Alpha Centauri B for any planet-induced wobbles claimed the star has an Earth-mass world in a scorching 3-day orbit, though that borderline detection has yet to be confirmed by other competing groups. Even if that world does exist, it would be far too close to its star to be imaged by ACESat.
“We’re already lucky to have Alpha Centauri right next door, but we’ll have to be even luckier for a small mission like ACESat to find anything there,” says Shawn Domagal-Goldman, a research scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “We’re looking for a world that, like Earth, could harbor a global biosphere we can detect across interstellar distances. That’s the jackpot, and each star you look at with your telescope is effectively one chance of hitting it. This type of mission targeting just one star system is the equivalent to the low-cost wager of buying just one lottery ticket.”
Belikov suspects the odds might be significantly better than that, pointing out that some recent estimates suggest about half of all stars may harbor rocky, potentially Earth-like planets in their habitable zones. “That implies there might be something like Earth around at least one of these stars,” Belikov said. In his view, ACESat’s chances for finding another pale blue dot are “probably higher than Columbus had of discovering America.”