'Old Earths': The Search for Ancient and Habitable (But Dying) Exoplanets

Illustration depicting the life cycle of Sun-like stars. Billions of years from now, our own Sun will expand into a red giant star, scorching any life that exists. Image Credit: ESO/M. Kornmesser

Illustration depicting the life cycle of Sun-like stars. Billions of years from now, our own Sun will expand into a red giant star, scorching any life that exists. Image Credit: ESO/M. Kornmesser

Researchers at Cornell University are taking a new approach to the search for alien life: looking for habitable planets older than Earth, “old Earth analogs,” which may be nearing the end of their habitable lifetimes. Astronomers would search for biosignatures from worlds much older than Earth, where lifeforms are dying off due to circumstances such as the planet’s star expanding in its old age, gradually heating the planet to a point where life is no longer possible.

Billions of years from now, our own Sun will meet this fate, expanding into a red giant star and scorching, or even engulfing, the Earth. In that sense they are searching for “future Earths”—planets already experiencing what Earth will far in the future.

The star 61 Vir, considered the most likely to have an "old-Earth" exoplanet orbiting it. Photo Credit: Kevin Heider

The star 61 Vir, considered the most likely to have an “old Earth” exoplanet orbiting it. Photo Credit: Kevin Heider

The research, conducted by Jack O’Malley-James, of the Institute for Pale Blue Dots at Cornell University, and his colleagues, calculates the chances of discovering an old Earth analog nearing the end of its habitable lifetime. The new paper, “In Search of Future Earths: Assessing the possibility of finding Earth analogues in the later stages of their habitable lifetimes,” will be published in the journal Astrobiology.

What are the chances of finding any old Earth analogs, if they exist? Smaller rocky planets are now thought to be quite common in the galaxy, thanks to discoveries from the Kepler Space Telescope and other telescopes. Plus, up to one-third of main-sequence stars are thought to be in the latter stages of their evolution, increasing the odds that some of them would have “Earths” which are also much older than our own planet.

Of course, the closer one of these planets might be, the easier it would be to study it and learn about its composition, atmosphere, and more. There are six stars similar to the Sun within 10 parsecs of Earth (1 parsec = 3.26 light-years). According to the new paper, the star 61 Vir would be the most likely to have an older version of Earth orbiting it. The postulated planet would be at a similar stage in Earth’s future, where temperatures have increased too much for anything to survive except microbes. The only life left on this world would likely be a declining microbe population.

Illustration of the habitable zone (in blue) for different masses of stars (our Sun = 1). As our Sun turns into a red giant star, the habitable zone moves outward. Image Credit: Wikimedia

Illustration of the habitable zone (in blue) for different masses of stars (our Sun = 1). As our Sun turns into a red giant star, the habitable zone moves outward. Image Credit: Wikimedia

To come to these conclusions, O’Malley-James calculated the location of the habitable zone for each of the six stars over their lifetimes. Then, the hypothetical planets in each system were placed at a distance where the planet could remain habitable for billions of years. By comparing the predicted incoming and outgoing radiation from the star(s), the temperature changes on the planets over the main sequence lifetime of the star could then be modeled.

A bit farther out, there are 276 stars similar to the Sun within 100 parsecs. About half of those are estimated to be more than six billion years old. By O’Malley-James’ calculations, there should then be approximately 11 old Earths within that distance from Earth.

But it may not be that simple; according to results from another study, smaller rocky planets like Earth are most likely to form less than one astronomical unit (AU), the distance between the Sun and the Earth, from their stars. But the continuously habitable zone of the six stars that O’Malley-James and his colleagues studied tended to be farther out from the stars than that, meaning that there might be just one old Earth in the near solar neighborhood, or maybe even none at all.

“It turned out that these planets are probably not that common at all, so in reality any habitable planets in the 61 Vir system will probably not resemble an older version of Earth,” O’Malley-James said. “This study highlights that finding replicas of our own world, in terms of the diversity and complexity of life, is going to be a much harder task than simply finding life.”

Artist's conception of exoplanet V 391 Pegasi b as it becomes engulfed by its expanding, dying red giant star. Earth faces a similar fate billions of years from now. Image Credit: HELAS, the European Helio- and Asteroseismology Network

Artist’s conception of exoplanet V 391 Pegasi b as it becomes engulfed by its expanding, dying red giant star. Earth faces a similar fate billions of years from now. Image Credit: HELAS/European Helio and Asteroseismology Network

It should be remembered, however, that the close solar neighborhood examined by O’Malley-James and his team are a very tiny percentage of all of the billions of stars in the galaxy overall. There could be many old Earths out there, but most might simply be too far away to study in any detail.

It is also thought that some “super-Earths,” rocky exoplanets larger than Earth but smaller than ice giants like Neptune, might be able to support some form of life. Current studies suggest that they might be even more numerous than smaller rocky planets. Computer simulations indicate that some of them might be true water worlds, completely covered by oceans.

Future space telescopes, such as TESSJWSTWFIRST, and PLATO, could help to find some of these ancient worlds, however, and characterize their compositions and atmospheres, as well as look for potential biosignatures.

As previously reported by AmericaSpace, the oldest known exoplanetary system found so far is Kepler-444, with five known planets, all smaller than Earth, and estimated to be 11 billion years old. Unfortunately, these worlds are most likely uninhabitable, already being scorched by their star in their small, tight orbits. The discovery does show, however, that very ancient Earth-sized worlds are out there. Any habitable exoplanets that old would have had much more time than even Earth to possibly develop a wide variety of life forms. There might even be worlds which are more biologically active than our home planet. What would life look like on a planet more than twice as old as Earth?

Indeed, astronomers now estimate that there are billions of smaller, rocky planets like Earth in our galaxy alone. Some will have conditions favorable for possible life, and some will not, but it increases the chances that there are indeed some older versions of Earth waiting to be discovered—worlds where life may have flourished for billions of years, but is now dying out. On some others, though, life might be thriving still on these older cousins of Earth, ones which have not yet been eradicated by their aging, swelling stars.

 

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15 comments to ‘Old Earths’: The Search for Ancient and Habitable (But Dying) Exoplanets

  • Gary Church

    There is only one technology needed for star travel to the living worlds we will almost certainly soon find. Freezing people and then reviving them. There are no real showstoppers in developing this technology as utilizing microwaves while freezing has already proven effective in preventing much of the cell damage associated with freezing. It is a case of technology long predicted becoming a reality- as Jules Verne predicted submarines and heavier than air flight. How to push sleeper ships to a percentage of the speed of light for centuries long voyages has been known for half a century- by using the most powerful device ever created- the H-bomb. Freezing people is also the ultimate disruptive technology. Nobody is going to let their loved ones die of disease or old age if they can be frozen till a cure is found. Such a revivable freezing procedure will be the most important event in the history of mankind. Everything will change- including our ability to travel to other stars.

  • In addition to Kepler 444, there is also a pair of planets known to be orbiting Kapteyn’s Star which has an age in excess of 10 billion years. The inner known planet, Kapteyn b, made headlines last year as being the oldest known potentially habitable planet.

    http://www.drewexmachina.com/2014/06/06/habitable-planet-reality-check-kapteyn-b/

    Although Kapteyn b is probably more likely to be a mini-Neptune than a terrestrial planet and its potential habitability overstated, this and future finds orbiting ancient stars would certainly be worthy of a closer look to gain insights into the potential habitability of old planet systems.

    • Gary Church

      In terms of humans migrating to other stars habitable planets may not be what to look for. Gerard K. O’Neill questioned whether a planet was even the best place to colonize. Artificial spinning hollow moons using asteroid material and comet water and volatiles are all that is necessary to sustain potentially billions, even tens of billions of humans.

      We need to be able to find star systems with asteroid belts and comet repositories like our Kuiper belt with over 10,000 icy bodies over 10 miles in diameter (if I recall). Using the energy of a sun to mass produce water filled hollow moons miles in diameter would actually serve as a better place than a new planet with very little chance of being exactly like Earth.

      We don’t need to find systems with planets- we need to find systems with resources.

      • Gary,

        I’m not advocating any position one way or the other about colonizing extrasolar planets nor is my comment meant to imply endorsing any position. Considering that the technology for practical interstellar travel (or even building “artificial spinning hollow moons”, for that matter!) doesn’t even exist, such discussions about the pros and cons of either tactic are premature, at best.

        Besides, a “habitable planet” as defined by scientists is not the same thing as a planet that is capable of supporting human life. A “habitable planet” is typically defined by scientists as being a world with conditions that can support the presence of liquid water over most of its surface much like the Earth does. In order to maintain those temperatures, the vast majority of “habitable planets” would have atmospheres with levels of greenhouse gases like CO2 that would be toxic to terrestrial animal life including humans(although any native life forms that evolved to tailor their biology for that environment would thrive). And it is by no means certain that the subset of “habitable planets” that do not have toxic levels of CO2 in their atmosphere would support the development of photosynthetic life forms that produce enough oxygen for us to breath. There are many forms of autotrophism (including forms of photosynthesis) that do not produce oxygen.

        The bottom line is, planets that are capable of supporting human beings without some sort of assistance are likely to be a small fraction of habitable worlds and will be exceptionally rare. With this in mind (combined with the fact that there is no practical way to reach any of these extrasolar worlds anytime in the foreseeable future),the reason why scientists today are looking for “habitable planets” is *NOT* to find a place for humans to colonize, it is to find other examples of habitable worlds so that we can gain a greater understanding about our planet and life in general.

        • Gary Church

          Considering that the technology for practical interstellar travel (or even building “artificial spinning hollow moons”, for that matter!) doesn’t even exist, such discussions – are premature, at best.

          “-the reason why scientists today are looking for “habitable planets” is *NOT* to find a place for humans to colonize-”

          And this is why the public does not care about space. You go ahead and shut down anyone with something different to say- good luck with that.

          • I am not shutting anyone down. I am responding to a common misconception which you seem to share about why scientists, like those discussed here, are interested in studying potentially habitable extrasolar planets… and, at this early stage, it has little to do with finding new worlds we can colonize since a) we have no practical means of reaching them in the foreseeable future and b) they are highly unlikely to support human life anyway. If anything, these facts lend support to your position advocating the building of spinning space colones like those envisioned by Gerard O’Neil in our own solar system (although the technology to build such colonies does not currently exist either, although it is certainly closer at hand).

            • Gary Church

              “-no practical means of reaching them in the foreseeable future-”

              “-the technology to build such colonies does not currently exist either-”

              It is a “common misconception” that we do not have the means to send a spaceship to another star. Freeman Dyson disagrees with you. H-bombs work. As I stated, only one technology is missing and it is not warp drive. It is a far less challenging matter of lowering and raising the temperature of a human body without damage.

              As for the technology to build such colonies not existing….puh-leez. We build thousand foot 60,000 ton(empty)supertankers on Earth and in zero gravity that 60,000 tons becomes a simpler engineering problem. We build Dubai towers and Hoover dams and bridges miles long. The technology does not currently exist. Puh-leez.

              So stop shutting other people down.

              • You seem intent on debating a subject that has absolutely nothing to do with either this article or my original comment… so I will give you what you seem to want: Technology is a body of practical knowledge needed to build and use devices that serve a purpose. Possessing the H-bomb and having the notion of harnessing its power as a means of propulsion is not the same as having the technology to make star travel feasible. There are a whole range of practical engineering problems that need to be solved before that idea can be made into a reliable device capable of propelling a craft across interstellar distance. By definition, that means we do not have the technology for star travel despite all the great ideas that exist. Based on everything I have read over the last forty years on interstellar travel, Freeman Dyson realizes this as well.

                Knowing how to build large structures like supertankers, towers, dams and bridges is not the equivalent of having the technology needed to build huge space colonies like those envisioned by Dr. O’Neil some forty years ago. While there are certainly skills and techniques used in the designing and building of large Earthbound structures that will be used in building such space colonies, there are numerous practical problems that need to be addressed to build and maintain such a structure ranging from the extraction and movement of all of the required resources to maintaining a self-sustaining habitat with a volume measured in cubic kilometers. While there are certainly many great ideas on addressing those problems out there, there has been no practical application of many of them. By definition, that means we do not currently have the technology to build huge space colonies.

                Finally, discussing the facts and expressing an opinion contrary to your own is not “shutting other people down”. To the contrary, it is at the heart of any intellectual debate. You believe that we have the technology to make interstellar travel feasible and to build huge space colonies. I do not agree.

                • Gary Church

                  My mistake was replying to your comment. It won’t happen again. You commented on the habitability of planets and I replied with my opinion on habitability in general. No “debate.”

                  You then talked down to me as much as possible using exclamation points, asterisks, and caps lock, and dismissed my points as essentially nonexistent. Then you denied my comment had anything to do with the article or your comment- which was a lie. Habitability?

                  Then you spoke for someone else I had used as a reference;

                  “Freeman Dyson realizes this as well.”

                  Not impressed. If that is not “shutting someone down” then I don’t know what is. You don’t agree with much of anything do you?

                • Neil

                  Andrew. I admire you for attempting to rationally debate the article with Mr Church however as you’ve discovered he has only one viewpoint, that of his own, and debate on the merits or otherwise appears pointless. Also I’ve noted that there is often a profound disconnect between said gentleman’s discussion and the article posted.
                  Cheers

                  • Gary Church

                    One of my fan club. This one is a rabid NewSpace sycophant who cannot stand it when someone dares to criticize his private space god Musk. So he has to “admire” anyone that disagrees with me. Disgusting.

                  • Neil, I appreciate your supportive comment and should have known better than to start this “conversation” since I too have evoked the wrath of Mr. Church in the comments section of other web sites for no rational reason. And, as if on cue, he has sunk to personal attacks with the “NewSpace mob” label in rseponse to your supportive comment. I am sure I am next… 😉

  • michael

    I think any search for extraterrestrial intelligence (SETI) should prioritize its targets according to the star age. Older stars with life supporting planets raise the chance of ETI being developed enough to master interstellar communication or even travel.
    Any ETI being threatened by its home star’s expansion would attempt to move away, either to a more distant planet orbiting the same star or possibly to another nearby star. If they can do that, they might be more likely to be able to communicate with other interstellar civilizations as well. This raises the chance for us to detect their signals.
    What do you think would happen if they have discovered a way to travel to other stars, and we happen to be near enough for them to reach us?

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