If we are to find life beyond Earth, we need to be explorers, not hunters

July 22, 2015 by Duncan Forgan, The Conversation
What secrets will space reveal? Credit: AstroStar

The news that the search for extraterrestrial intelligence is to receive increased funding and data through the $100m (£64m) Breakthrough Listen project is welcome news for astrobiologists like myself. Launched by Stephen Hawking, it particularly helps to allay growing concerns in the field about having too narrow a focus in our search for life in the universe.

Last week I attended the Pathways Towards Habitable Planets conference in Switzerland, where leading scientists in the search for habitable planets shared their results and ideas for the future. What was especially interesting was the relatively strong consensus on the problems with our definition of the habitable zone – the area around a star which is neither too hot nor too cold for orbiting planets to support liquid water on the surface. Even its name is misleading, as we'll see in a moment. If we aren't careful, obsessing about this zone could prevent us from reaching our ultimate goal of finding extraterrestrial .

For as long as we have considered planets orbiting other stars, we have speculated over their propensity to host living organisms in the way that the Earth does. The habitable zone concept has helped astronomers to define where, in all those quintillions of acres of galactic real estate, we should search for planets that might be inhabited.

It may seem sensible to look for in regions where any Earth-like planet would have liquid water on the surface. Liquid water is an essential solvent for the chemical reactions that Earth biology relies on. If we find planets with liquid water, they satisfy a key criterion for being conducive to life as we know it.

Yet being in the zone neither automatically means that a planet will have water, nor that it could support life. It needs to have a "healthy" atmospheric composition – usually assumed to mean similar to Earth's – and ideally a healthy magnetic field to shield it from high-energy particles belched forth by its parent star.

We might also demand that the planet's orbit and rotation is stable and that any planetary neighbours kindly leave it alone. We don't have enough data on the planets we have found to date to know if they meet all these criteria. Even if we did, we would most likely have to run a sophisticated computer simulation to model their climate before we could determine what conditions were really like on the surface.

These difficulties with the definition of the habitable zone can lead to astronomers and astrobiologists coming a cropper when speaking to the press. When a press release announces the detection of "a planet in the habitable zone", the general public reads "a habitable planet". It's this confusion that prompted the discussion at the Pathways conference on whether we should change the zone's name to something else – perhaps the surface liquid water zone, or the temperate zone.

Could Titan’s methane lakes host life? Credit: manjik

Beyond this, there are other problems with the concept. Perhaps life doesn't require surface liquid water to survive at all. Some have speculated that the liquid hydrocarbons on Titan, Saturn's largest moon, could be a solvent for a very different form of life, for example.

Other moons in our solar system, such as Europa and Enceladus, meanwhile, appearto have subsurface even though they reside outside the traditional habitable zone. The tidal heating they receive from their host planets is enough to make beyond the habitable zone, if that's not too confusing. The more we learn about other planets, the more the simplicity of the habitable zone's definition begins to look dangerous.

The need for focus

So why have astronomers persisted with the concept? The real reason is target selection. There are lots of stars in the Milky Way – and we now know of lots of planets surrounding them. Astronomers have limited resources, and not all astronomers want to search for biospheres.

Because we can only observe a few targets, we choose the ones that we think have a higher chance of yielding signs of life. Depending on how you detect them, most candidates are merely silhouettes on a star's surface or wobbles in a star's orbit. If we're lucky, some are both – or we have managed to discover some information about molecules in their atmospheres using transit spectroscopy, which is the study of the light the planet reflects from its parent star.

The next generation of exoplanet observations is designed to ensure we glean the maximum amount of information about as many planets as possible. This is in advance of the coming extremely large telescopes, which may be able to directly image any "Earth-like" planets nearby.

We should be exploring every kind of planet, not just hunting for ‘blue marbles’ like our own. Credit: Fisherss

Yet this isn't an excuse for going down the wrong path. It might be tempting to rush to the end of the search – hunting exclusively in the habitable zone – but we might be rushing to the wrong end. Consequently many scientists are saying we shouldn't be looking for things that look like life, but merely things that look anomalous and can't be explained by geochemical, non-biological processes.

The weird blooms of methane in Mars' atmosphere pointed towards life, for instance. This turned out to be something of a false alarm, since they can also be explained without requiring organisms, as can many other potential signs of life on the planet. Frustrations aside, such anomalies are still worth exploring. The more we find, the more likely we are to find one caused by organisms.

I'm pleased to say that Breakthrough Listen is in the spirit of this approach. It will focus on sifting data from radio and infrared telescopes for signs of extraterrestrial intelligence. It will not restrict its focus to zones, particular conditions, or even planets at all, but scan more widely to look for signals that can't be explained by natural phenomena.

Every planet we find and learn about – even hellish worlds such as Venus, or gas giants such as Jupiter – is a piece of the puzzle of how form and evolve. They all help us learn how biospheres are born and how common or rare we really are. As we put our blue marble into ever clearer context, it's my fond hope it will help us appreciate and cherish our singular, complex, beautiful world all the more.

As Franck Selsis, a leading figure in finding and characterising potentially habitable worlds, said at the Pathways conference, "Perhaps the best strategy is to have no strategy – except to simply explore."

Explore further: Planets in the habitable zone around most stars, calculate researchers

Related Stories

Kepler's six years in science (and counting)

May 13, 2015

NASA's Kepler spacecraft began hunting for planets outside our solar system on May 12, 2009. From the trove of data collected, we have learned that planets are common, that most sun-like stars have at least one planet and ...

What is the habitable zone?

June 30, 2015

The weather in your hometown is downright uninhabitable. There's scorching heatwaves, annual tyhpoonic deluges, and snow deep enough to bury a corn silo.

Finding infant earths and potential life just got easier

December 4, 2014

Among the billions and billions of stars in the sky, where should astronomers look for infant Earths where life might develop? New research from Cornell University's Institute for Pale Blue Dots shows where - and when - infant ...

Habitable zones

August 22, 2011

(PhysOrg.com) -- The "habitable zone" is the region around a star where a suitable planet could sustain the conditions necessary for life. Most astronomers take it to be the region where the balance between stellar radiation ...

Recommended for you

How hot are atoms in the shock wave of an exploding star?

January 21, 2019

A new method to measure the temperature of atoms during the explosive death of a star will help scientists understand the shock wave that occurs as a result of this supernova explosion. An international team of researchers, ...

New eclipsing cataclysmic variable discovered

January 21, 2019

Using the Mobile Astronomical System of Telescope-Robots (MASTER), an international team of astronomers has detected a new eclipsing cataclysmic variable. The newfound object, designated MASTER OT J061451.70–272535.5, is ...

The disintegrating exoplanet K2-22b

January 21, 2019

Exoplanet surveys have yielded many surprises over the years, and the discovery of "disintegrating" exoplanets was one of them. These are planets that produce asymmetric shapes in the dips of the light curves seen as they ...

Total lunar eclipse woos sky watchers

January 21, 2019

An unusual set of celestial circumstances came together over Sunday night and the wee hours of Monday for sky watchers in Europe, Africa and the Americas, where the moon was fully obscured before lighting up again with a ...

Making stars when the universe was half its age

January 18, 2019

The universe is about 13.8 billion years old, and its stars are arguably its most momentous handiwork. Astronomers studying the intricacies of star formation across cosmic time are trying to understand whether stars and the ...

3 comments

Adjust slider to filter visible comments by rank

Display comments: newest first

antialias_physorg
3.7 / 5 (3) Jul 23, 2015
"...we need to be explorers, not hunters."

Fact: We can't be anything other than the life we are.
Fact: Our choices and what we do are not the definition of life, and so cannot define us, as we are life.

Did you even read the article?
jordonrodriguez57
5 / 5 (1) Jul 23, 2015
"...we need to be explorers, not hunters."

Sam and Dean would not like this article.
Crux315
3.7 / 5 (3) Jul 26, 2015
DavidW, did you even read the article? It's talking about "exploring" for planets that could be the hosts of life were are unfamiliar with rather than "hunting" for "blue marbles in the Habitable zone."

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.