Is Alpha Centauri the right place to search for life elsewhere?
It sounds like science fiction. From the people who brought you the project Breakthrough Listen to search for extraterrestrial life, comes a new research program that's looking at sending a tiny spacecraft to the nearest stars.
The US$100 million plan is to push these probes out at speeds up to a fifth of the speed of light. To do this would require huge technological innovation, but it's certainly not beyond the bounds of possibility.
But if the project is to bear fruit, where should these minute spacecraft be sent? The first suggested target is the Alpha Centauri system, the closest stars to the solar system.
The first stop on an interstellar journey
Alpha Centauri appears a single star when seen with the unaided eye, and is the third brightest star in the night sky. But when observed through binoculars or a telescope, you can see the star is double – a binary star system.
The two bright components, Alpha Centauri A and B, are similar to our sun. One (A) is a bit brighter and bigger than our star and the other (B) a little fainter and smaller.
They move together in lockstep, orbiting their common centre of mass roughly every 80 years. As they do, they follow an elliptical path, with their closest approach (periapse) roughly 11 times further than the Earth is from the sun.
And the two are not alone; they are accompanied by Proxima Centauri. Proxima is a dim red dwarf star, about an eighth the mass of the sun.
It currently lies a little closer to the Solar System than the other two, and so holds the distinction of being the closest star to the sun. Despite this, it is so dim that it is far too faint to see with the unaided eye.
Sunlike stars, but where are the planets?
As our nearest stars, the Alpha Centauri system has been an obvious target for the search for exoplanets. Dedicated search programs, such as the Mt John Alpha Centauri Project, look at the stars every single clear night, trying to uncover even the slightest hints that they might host planets.
Other programs on the world's largest telescopes observe less frequently, but with exquisite precision.
The result? Well, a few years back, the discovery of a planet around Alpha Centauri B was announced to much fanfare.
Had that planet been real, spotting it would have been groundbreaking. A tiny, broiled world, skirting the top of the star's atmosphere.
Sadly, though, as more observations have come in, the planet's existence has fallen into doubt. An extensive reanalysis has effectively added it to the pile of planets that never were.
So why go to Alpha Centauri?
Given that Alpha Centauri is currently viewed as a planet-free zone, why would we want to go there?
Probably the first and foremost reason is that it is nearby, closer than any other star. If the new spacecraft were to achieve the proposed fifth of the speed of light, it would only take 21 years or so to get there (depending on the time taken to accelerate). That is far shorter than the travel time to any other known star.
Sending our first probes out to Alpha Centauri would mean we get our first closeup look at another star, far sooner than for any other known star. We'd also get a two-for-one peek, whizzing past Alpha Cen A and B up-close and personal.
We'd even get a wealth of data on Proxima Centauri, thrown in for good measure. We couldn't get all that close, though, since these spacecraft are going to be more like bullets than racing cars, fired outward from Earth.
And if there are planets around these stars, then we'd see them. In fact, if there are planets there, they'd likely be found before our tiny explorers reach the area (given the rate at which our techniques and telescopes are improving).
So we'd be able to let the spacecraft know, and plan its observations to take advantage.
Looking further afield
Let's say the mission to Alpha Centauri is a success. Where should we go next?
One exciting target lies just a little further away than the stars in our table (above) and that's Epsilon Eridani. At just 10.5 light years distant (a travel time of a meagre 55 years for our tiny explorers), it is still one of our nearest neighbours.
Where Alpha Centauri is a multiple star system, with its sun-like stars so close as to render the formation of truly Earth-like planets challenging, if not impossible, Epsilon Eridani is a solitary wanderer, just like the sun.
A little smaller and dimmer than our star, it is known to have two disks of debris orbiting around it. Again, this is just like our sun. The inner disk looks a bit like our asteroid belt, around the same distance, around the same size.
Observations have revealed the presence of at least one massive planet in the system, moving on an orbit just outside the inner asteroid belt. Just like Jupiter in our solar system. There may well be others, lurking and awaiting discovery.
If we want to explore a system that might just be uncannily like our own, then Epsilon Eridani is probably the place we should look. But with a travel time of more than 50 years with the proposed technology, it makes sense to shoot for the closest stars first.
All aboard for Alpha Centauri!