Why all the fuss over Alpha Centauri Bb?

The astronomical community is abuzz with news of the discovery of an Earth-sized world orbiting another star. Exciting this may be, but this particular extra-solar planet (aka. an “exoplanet”) certainly isn’t Earth-like. It orbits its parent star ten-times closer than the planet Mercury orbits our Sun. It zooms around the star every 3.2 days and has a surface temperature in excess of 1,230 degrees Celsius. Although it is most likely a rocky world, its surface will be a molten hell and very hostile to life.

So what’s the fuss all about? Small alien planets are known to orbit other stars in the galaxy – in fact, there’s thought to be tens of billions of them out there, many of which are likely more habitable than the exoplanet described above – and NASA’s Kepler mission has spotted candidate worlds as small as Mars. Surely this new world is just another addition to the weird and growing menagerie of exoplanets?

Actually, this most recent discovery may well be one of the most significant finds in the burgeoning science of alien planet-spotting. This exoplanet has been discovered right on our cosmic doorstep – a mere 4.2 light-years away.

Tugging, not transiting

Interplanetary space is big. But interstellar space is mind-bogglingly vast. 4.2 years is the time it would take for light to travel from our Sun to the next nearest star system – that’s a whopping 25 trillion miles. In galactic scales, that’s a mere stone’s throw from home. In human scales, that’s a huge, impenetrable void quarantining mankind from the rest of the cosmos… or is it?

Alpha Centauri is a binary star system (where two stars – Alpha Centauri A and B – orbit one other, locked in a gravitational dance) that has, until recently, shown few signs of any planets orbiting either star. A third star, that is thought to be weakly gravitationally bound to the Alpha Centauri system, called Proxima Centauri, is located a little closer to Earth, but as it is a tiny flaring red dwarf the chances of habitable planets orbiting it are slim.

The apparent dearth of exoplanets around Alpha Centauri changed when astronomers using the European High Accuracy Radial velocity Planet Searcher (HARPS) telescope detected the very slight gravitational tug of a small world orbiting Alpha Centauri B, a star not too dissimilar to our Sun. The new exoplanet has been designated Alpha Centauri Bb.

When hunting for exoplanets, astronomers have several methods by which the distant world may be directly or indirectly detected. The two most frequently used methods are known as the transit method and the radial velocity method. Kepler, for example, uses the transit method – the extremely sensitive space telescope constantly watches one patch of sky, waiting for exoplanets to pass directly in front of their stars, causing a very slight dimming of starlight. This is known as a “transit”. However, the transit method is only effective for star systems that are “edge-on” from our perspective. Although hundreds of exoplanetary candidates have been found, countless others have different orbital configurations, thus requiring astronomers to employ the “radial velocity” method.

Surveys such as HARPS can detect very slight wobbles in stars’ positions. As a planetary body orbits its parent star, it pulls slightly on the star, causing a slight shift in position. This shift can be detected in the light received from the star – its wavelength gets slightly stretched (redshifted) as the star is pulled away and compressed (blueshifted) when pulled toward the observer. Small worlds orbiting close to their parent stars can therefore be indirectly observed.

The habitable zone doesn’t mean it’s habitable

As we find more and more alien worlds, patterns are beginning to form. One pattern is that small rocky worlds usually exist in multi-planetary systems. In other words, statistically speaking, if you find small exoplanet, there will be, in all likelihood, more orbiting the same star. Alpha Centauri Bb may have been discovered on its lonesome, but that doesn’t mean it’s alone; there may be an entire system of rocky worlds. This is why Alpha Centauri Bb is so exciting; it may be an indicator that more small rocky worlds exist in the orbit of the star.

All exoplanetary detection methods have their strengths and weaknesses, and for the radial velocity method, its strength is in the fact it can detect exoplanets that never transit their stars from our point of view. But it can only detect small worlds that have very compact orbits (or it can detect massive gas giant worlds with wider orbits). Although it was difficult to detect, Alpha Centauri Bb had a compact enough orbit to impact its star in a measurable way. Unfortunately, the same method can’t be used to detect other hypothetical Earth-sized worlds in orbits more suitable for life to thrive.

This hypothetical Earth-sized world would need to orbit within a region surrounding the star where the conditions are just right for liquid water to exist on its surface. Too close, and the water evaporates; too far and the water freezes. Liquid water is a prerequisite for life (as we know it), so this is one key attribute the world must have – it must orbit within its parent star’s habitable zone. But that’s not the “silver bullet”, for a genuine Earth-like world to be confirmed, we’d need to construct extremely powerful telescopes capable of analysing the world’s atmosphere.

From a distance, Earth and Venus are both “Earth-sized” and both orbit the Sun (more or less) inside our star’s habitable zone. Yet Venus is a hellish planet blanketed in a choking atmosphere that is hot enough to melt lead. The Venus/Earth contrast is stark and we’d need to keep this in mind before declaring any exoplanet remotely “habitable”.

We need to be ready

Astronomers may not find evidence of more exopanets orbiting Alpha Centauri B for some time, but as a species we need to get ready for the potential of an Earth 2.0 existing in the star system just next door.

In a thought-provoking Discovery News interview with Icarus Interstellar co-founder and president Dr Richard Obousy, the fact that we are beginning to uncover the existence of exoplanets around neighbouring star systems reveals opportunities for interstellar exploration. “The discovery of an Earth-sized world that is so close to us, when measured in interstellar distance scales, means that when we select target solar systems for exploration, we may not have to choose systems that are prohibitively far from our own solar system,” he said.

Icarus Interstellar Inc is a non-profit group of scientists and engineers working on developing the technologies for an interstellar probe that can arrive in a target star system within 100 years after launch. But the biggest issue interstellar advocates have faced is the lack of potential targets. Now we’ve found Alpha Centauri Bb. It may not be suitable for searches for extraterrestrial life and it may not be a viable Earth analogue where we could set up some kind of colony, but it is interesting that more worlds may exist around the star.

“The discovery of (Alpha) Centauri Bb isn’t quite as momentous as this owing to its proximity to the star, but this discovery is a warning shot that the news is coming a lot sooner than we think,” said Robert Freeland, Icarus Deputy Project Leader. By Freeland’s reckoning, the world’s space agencies need to be ready in the event of the discovery of a bona fide Earth-like exoplanet orbiting a star next door. Should this happen, the world may demand that we send a starship to investigate.

Although huge technological advances in space propulsion technologies will be required, attention needs to be paid to groups like Icarus Interstellar. Once mankind becomes interplanetary, colonising other worlds in the Solar System, it would stand to reason that we could eventually become an interstellar species. This may be a science fiction dream, humanity may never, realistically, be able to realise a future amongst the stars. But it is my opinion that interstellar travel will be a worthy challenge.

However, before we can entertain this notion, we’ll have to work together as a planet – a challenge that may be greater than attempting to visit Alpha Centauri B.

[Image courtesy of Adrian Mann]

Ian O’Neill is Space Science Producer for Discovery News. He is also the founder and editor of space blog Astroengine.

Follow him on Twitter: @astroengine