National Geographic Daily News
Photo of Alpha Centauri B.

This illustration shows the planet orbiting the star Alpha Centauri B, the lightest exoplanet ever discovered around a sunlike star.


Sarah Fecht

for National Geographic

Published January 17, 2014

Planet hunters have always been keen to find Earth's twin, but an astrobiology team now suggests that "superhabitable" planets may be even better places to look for alien life.

Since 1995, astronomers have detected more than 1,000 worlds orbiting nearby stars, sparking a race to find the one that most resembles Earth, blessed with oceans and an oxygen-rich atmosphere. That's because Earth is the only place in the universe where we know that life has evolved. (See: "More Than 1,000 Potential New Planets Found.")

In the journal Astrobiology, however, researchers René Heller of Canada's McMaster University and John Armstrong of Weber State University in Ogden, Utah, calls that idea too Earth-focused. "From a potpourri of habitable worlds that may exist, Earth might well turn out as one that is marginally habitable, even bizarre from a biocentric standpoint," they write.

Instead, they suggest that astronomers should focus their planet hunting on worlds that might harbor conditions even more amenable to life. The authors dub these hypothetical worlds "superhabitable." (See "Think Outside the Box to Find Extraterrestrial Life.")

Their report adds to a chorus of voices in the planet-hunting community that have called for rethinking the idea of "habitable zones" where worlds that follow orbits friendly to oceans and life would exclusively exist.

Water Worlds

What characteristics might make a world superhabitable? Like all potentially habitable worlds, they should have water, agree Heller and Armstrong. But they list more than a dozen additional geological and atmospheric factors that could influence habitability.

For instance, older planets would presumably have had more opportunities for life to evolve. Larger worlds, ones up to three times as massive as Earth, might be more likely to have an atmosphere due to more volcanic activity, which releases gases.

Earth itself is thought to be located on the fringes of the habitable zone, they note, so maybe planets that are located nearer to the center of the habitable zone are more congenial to life.

Scientific Skepticism

Other scientists disagree about the usefulness of the concept of superhabitability. "A planet is either habitable or it's not," says atmospheric scientist Jim Kasting, who first introduced the concept of the circumstellar habitable zone, which defines a planet as habitable if it orbits its star at a distance where it's not too hot and not too cold for liquid water to form on the planet's surface.

Similarly, astrophysicist Steven Desch said that "calling a planet superhabitable is comparable to calling someone only a little bit pregnant ... Having more of what is needed for life, in my mind, doesn't make it more likely to have life."

But Ravi Kopparapu, a physicist at Penn State University, agrees with the authors that the "binary" habitable zone concept (either friendly to life or not) is too restrictive. Plenty of worlds within the habitable zone are unlikely to support life, while others—such as the icy moons of Saturn and Jupiter, which may have vast underground oceans—could potentially support life but fall outside the habitable zone. As scientists continue to discover a menagerie of exoplanets, considering more variables could help to prioritize which planets to target for follow-up.

Nevertheless, "there is a very good reason why the binary habitable zone concept is important and relevant," says Kopparapu. Currently, when astronomers discover a planet, all they can learn about it is its mass and radius, how much light it receives from its star, and occasionally the composition of its upper atmosphere. Until scientists develop the techniques to study a planet's surface features, tectonic activity, and geological composition, the habitable zone concept remains the best guess of its habitability, says Kopparapu.

Worlds Waiting

When NASA launches the James Webb Space Telescope in 2018, it may help scientists to take a closer look at a planet's atmosphere, detect if it has oceans, and analyze its chemical composition.

If superhabitable planets exist, and if we develop the means to find them, they may turn out to be more common than Earthlike planets—current studies suggest that super-Earths are more common than Earth-size planets (although those studies may be biased by the fact that it's easier to spot larger planets).

The concept of superhabitability could broaden our chances of discovering life on other worlds, Kopparapu says, "because it opens up the possibility that there may be some super-Earth planets with appropriate conditions for life ... I think it is noteworthy to consider these planets for future habitability studies."

Follow Sarah Fecht on Twitter.

Bastien Rene
Bastien Rene

There's also another factor that I only too rarely see mentioned, the axis of a planet in the course of its rotation around its star.

Did you guys realize our Earth has an almost perfect axis, with a fitting rotation cycle ?

Try to picture yourself, if the Earth's rotation axis (not for the solar year, for the Earth's Day) was not 90° but 0° : one side of the planet would be scorched half of the year, and deep-cold frozen the other half of the year.

The only good solution for a planet to be life-friendly is to have our current axis. Maybe better than 90°, our axis : it allows for the seasons to change, which might encourage diversification and versatility.

Unless I am mistaken, it is currently impossible to measure, for the moment, a distant planet's axis tilting, sadly.

Nelly Tolbert
Nelly Tolbert

I think that we should stay on this planet and leave the others alone, because we destroyed this planet, so we should have to deal with the consequences.

David Finch
David Finch

Life to me is a force that is able to interact with and change its environment. It is the product of a very complex array of bio chemical interactions all of whihc generates energy flows which combine into a single flow that makes life what it is. And since bio chemical ractions that form life are dependent on the elemenat al make up of the form. And since there only a limited ways in which elements can be formed there will only be a limited way in which life can form. This is born out by the fact that mamals, reptiles, fish etx all have the same basic structure viz a ciruakatory system, a digestive system, a nervous stem and reproductive system. Thus I would sugest that life can only emerge of earth like planets and not any other. By this I speak of complex/sentient life forms. Microrganisms could form on planets different from our own. The aforegoing is true if the theory of evolution is correct!

John Canfield
John Canfield

Dear Ms. Fecht, I guess my first hurdle to get over would be: What could be more habitable than the only inhabited planet we know of so far? Thousands of stars investigated, hundreds of planets discovered, life on only one, our own, so far. 

Lee N
Lee N

And when someday we discover them, we can move to the planet and take it over from the creatures living on the planet. Then, we become bad aliens hahaha...

Dillip Kumar Mohanty
Dillip Kumar Mohanty

I believe life has different channel of manifestation. Yes, water, temp. and like others are mere a part of what life needs to exist in earth but these things show our limitation to understand the concept of life. 

Till we discover the true nature of life by creating a living organism from non-living things, we will be in may be/might be region. till then we can't say the complete parameters life requires for existence. In this scenario, I feel, superhabitability is a right concept for better understanding of the wider aspect of life.

Setya Caturcahaya
Setya Caturcahaya

I've seen at the time of clear sky in the early morning, something that initially thought a shining star that aligned with the constellation sagittarius, but after approximately one minute and the something light that suddenly moves straight and fast to disappear... wow,incredible...I do not know what it is but it made me felt lucky to see it.

Adithya Ravi
Adithya Ravi

Hypothetically: Isn't everything we talk about and believe, just earth-centric? Can living things not exist without water? On another planet might there not be something better that we do not know about? Wont the bodies of those on other planets depend on those resources present in there world?

Mireia Estrany
Mireia Estrany

Then, perhaps, is there fossils or inteligible life elsewhere? But, isn’t the emergence and maintenance of life a process of radical contingency? That is, is a unique and unrepeatable past totally necessary? Or does life emerge through space like mushrooms when some conditions are present? So, how many conditions are necessary: three, four, trillions, infinite? Only one, water or any sort of God? Is God the word that means infinite conditions, absolute necessity? Anyway, how did the life that emerge in a given conditions resist when switching to a different  moment? How does life resist time itself, the effects of entropy? Indeed, is it possible for human beings to recognise a simpler life than their own brain only? On the other hand, beyond likeness, is it possible to recognise a complex life than their brain, is this the extra-terrestrial life that some people are searching unsuccessfully? However, is there an origin of life or would it be as finding a cut in the material history of the universe, an infinite void that human language patches now? Along these lines, there is a peculiar book, a short preview in Just another suggestion, far away from dogmas or axioms.

Stephanie Freeman
Stephanie Freeman

I think you are probably correct about when they might find life, or at least, when they might know for a fact that it exists somewhere else other than our solar system. (They might discover life on Europa or another moon in the next 20-30 years, depending on project funding for probes.)

However, looking for liquid water is a good idea because water will almost certainly (99.9999%) be required for life to exist. This is because many of the elements that make up life here (or make the energy life needs to survive) are soluble in water. What that means is that it is easier to combine elements when water is present, so it's easier to create something (a life-form) when water is added to the mix. Every single life-form that has ever been found on Earth, from viruses to whales, including the extremophiles, has needed water for some process of survival. There is no reason to think that these physical laws would not be followed somewhere else.

Chriss Salim
Chriss Salim

In my opinion , if they will find life somewhere else (habitable planet) , it won't happen in our era . Now , they are searching way beyond our solar system . If we take a look about Earth and its geological aspects , it's a unique planet until this moment , who knows maybe they can find another planet with same geological factors .

But talking about geological factors is not specific because we are not sure if aliens need other factors to live (maybe they don't need water or O2 or . . .or . . .) and in this case all the concepts of "habitable planet" become useless , it's something we don't know .

John Canfield
John Canfield

@Setya Caturcahaya Dear Setyai, What a fascinating experience that must have been. As a possible explanation, did you know that Sagittarius is one of the centers of meteoric activity for Earth? If you were observing close to the radiant point (the center of activity) you might have observed an object coming from the center more in your line of sight, then as it moved toward the edge of the 'cone' it would appear to accelerate in that direction. This is only a suggestion--not all sightings of this nature are explained. It's just a calming thought. Yours, John

Denmark Bernardo
Denmark Bernardo

@Chriss Salim i was thinking the same way too, like maybe they have their own needs, maybe they do not need what us HUMANS need, therefore though water and O2 is not present on a planet/A body there could still be lifeforms.

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