Arizona State University
Whether there is life elsewhere in the universe is a question people have pondered for millennia; and within the last few decades, great strides have been made in our search for signs of life outside of our solar system.
NASA missions like the space telescope Kepler have helped us document thousands of exoplanets – planets that orbit around other stars. And current NASA missions like Transiting Exoplanet Survey Satellite (TESS) are expected to vastly increase the current number of known exoplanets. It is expected that dozens will be Earth-sized rocky planets orbiting in their stars’ habitable zones, at distances where water could exist as a liquid on their surfaces. These are promising places to look for life.
This will be accomplished by missions like the soon-to-be-launched James Webb Space Telescope, which will complement and extend the discoveries of the Hubble Space Telescope by observing at infrared wavelengths. It is expected to launch in 2021, and will allow scientists to determine if rocky exoplanets have oxygen in their atmospheres. Oxygen in Earth’s atmosphere is due to photosynthesis by microbes and plants. To the extent that exoplanets resemble Earth, oxygen in their atmospheres may also be a sign of life.
Not all exoplanets will be Earth-like, though. Some will be, but others will differ from Earth enough that oxygen doesn’t necessarily come from life. So with all of these current and future exoplanets to study, how do scientists narrow down the field to those for which oxygen is most indicative of life?
To answer this question, an interdisciplinary team of researchers, led by Arizona State University (ASU), has provided a framework, called a “detectability index” which may help prioritize exoplanets that require additional study. The details of this index have recently been published in the Astrophysical Journal of the American Astronomical Society.
“The goal of the index is to provide scientists with a tool to select the very best targets for observation and to maximize the chances of detecting life,” says lead author Donald Glaser of ASU’s School of Molecular Sciences.
The oxygen detectability index for a planet like Earth is high, meaning that oxygen in Earth’s atmosphere is definitely due to life and nothing else. Seeing oxygen means life. A surprising finding by the team is that the detectability index plummets for exoplanets not-too-different from Earth.
Although Earth’s surface is largely covered in water, Earth’s oceans are only a small percentage (0.025%) of Earth’s mass. By comparison, moons in the outer solar system are typically close to 50% water ice.
“It’s easy to imagine that in another solar system like ours, an Earth-like planet could be just 0.2% water,” says co-author Steven Desch of ASU’s School of Earth and Space Exploration. “And that would be enough to change the detectability index. Oxygen would not be indicative of life on such planets, even if it were observed. That’s because an Earth-like planet that was 0.2% water–about eight times what Earth has–would have no exposed continents or land.”
Without land, rain would not weather rock and release important nutrients like phosphorus. Photosynthetic life could not produce oxygen at rates comparable to other non-biological sources.
“The detectability index tells us it’s not enough to observe oxygen in an exoplanet’s atmosphere. We must also observe oceans and land,” says Desch. “That changes how we approach the search for life on exoplanets. It helps us interpret observations we’ve made of exoplanets. It helps us pick the best target exoplanets to look for life on. And it helps us design the next generation of space telescopes so that we get all the information we need to make a positive identification of life.”
Scientists from diverse fields were brought together to create this index. The formation of the team was facilitated by NASA’s Nexus for Exoplanetary System Science (NExSS) program, which funds interdisciplinary research to develop strategies for looking for life on exoplanets. Their disciplines include theoretical and observational astrophysics, geophysics, geochemistry, astrobiology, oceanography, and ecology.
“This kind of research needs diverse teams, we can’t do it as individual scientists” says co-author Hilairy Hartnett who holds joint appointments at ASU’s School of Earth and Space Exploration and School of Molecular Sciences.
In addition to lead author Glaser and co-authors Harnett and Desch, the team includes co-authors Cayman Unterborn, Ariel Anbar, Steffen Buessecker, Theresa Fisher, Steven Glaser, Susanne Neuer, Camerian Millsaps, Joseph O’Rourke, Sara Imari Walker, and Mikhail Zolotov who collectively represent ASU’s School of Molecular Sciences, School of Earth and Space Exploration, and School of Life Sciences. Additional scientists on the team include researchers from the University of California Riverside, Johns Hopkins University and the University of Porto (Portugal).
It is the hope of this team that this detectability index framework will be employed in the search for life. “The detection of life on a planet outside our solar system would change our entire understanding of our place in the universe,” says Glaser. “NASA is deeply invested in searching for life, and it is our hope that this work will be used to maximize the chance of detecting life when we look for it.”
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Hello, Pat Frank. Most of natural selection occurs in the egg? How can selection pressures influence eggs, or even zygotes? I was taught that environmental factors drive evolution by favouring some phenotypes over others. The reason that there are tigers is because there are certain evolutionary advantages to being a large striped beast with fangs and claws. None of these traits present advantages that are in any way apparent in the egg. (Which is the female gamete, incidentally. Sperm don’t have anything to do with evolutionary selection? Is that your theory?)
Ligers are sterile. Strictly speaking, they aren’t a separate, naturally occurring species, even if their parents are. Lions and tigers are separate species, but if ligers were fertile, lions and tigers would by definition be of the same species.
Most scientists reject intelligent design out of hand, because Science rejects supernatural causes. Of course, if you believe in supernatural forces, even if you can’t describe them reliably, you’re allowed to insert God into the mix. Which is what I do, although I emphasize that, to me, “God” is just a vague term that has no discretely defined meaning.
Y’know, Pat, at some point in the history of the Earth, inanimate chemicals must have combined to form animate organisms. This must have happened on a wide scale all over the globe. Why don’t we see this phenomenon today? Did it just stop as abruptly as it started? What I’m getting at here is that there are huge gaps in our knowledge, and I just fill ’em with God. The concept of God is like putty for cracks in theories. Hey, works for me.
Ian, you asked, “How can selection pressures influence eggs, or even zygotes?”
The egg and the uterus are complex and dynamic environments, Ian. That environment interacts wit, and responds to, the developing offspring, which responds in turn.
Genes in the developing organism turn on and turn off, and must do so in proper sequence, in part in response to environmental queues. If the turn on is too early or the turn off too late, the impact can be large.
Feedbacks upon feedbacks. An incorrect developmental environment can impact the developing conceptus. Most likely for the worse, possibly to the advantage of the emergent young.
Ligers are not sterile.
“but if ligers were fertile, lions and tigers would by definition be of the same species. ”
Not correct. Lot’s of related species can interbreed. They don’t typically because their breeding habits differ, not because their gametes are incompatible.
I coauthored a book chapter on chiral take-over and chemical biogenesis, Ian, On One Hand But Not The Other: The Challenge of the Origin and Survival of Homochirality in Prebiotic Chemistry.
It needed to happen only once. Once a self-contained reproducing system emerged, it would take over.
Earth is now a very, very different place than it was 4 billion years ago. More of the surface was ocean than now. Continents were small granite plates. Tectonics was intense with extensive and hyper-vigorous chemical smokers. The atmosphere was 60 bars of CO2, and large reefs of ferrous iron were abundant.
None of that is present on today’s Earth. There’s no reason to expect similar observables.
Okay, Pat, you’ve got me there when you point out that the Earth’s environment is dramatically different from what it was when life first arose.
Or course, my doubts about Evolution are originally intuitive. It just doesn’t seem possible, for example, that the process that has produced animal intelligence is itself unintelligent, which is why I must believe in an intelligent designer. The universe is not a giant self-winding watch that just happened to produce my cousin Dave, who could eat live worms. I can no more believe that hummingbirds and bullfrogs and Shetland ponies are the result of random collisions of atoms than I can believe that St. Paul’s cathedral is a natural rock formation.
A lot of times you run into people who are aggressively, angrily atheist. (Richard Dawkins and Sam Harris are prime examples of these gentry.) Obviously the strongest argument for the existence of God is that the world exists: Somebody must have made it, surely. The Theory of Evolution gives atheists a trapdoor through which they can slip to avoid the otherwise obvious proposition that the World was created.
Ian what gain is there in saying that a hard-to-understand system can be explained by supposing an impossible-to-understand system?
That’s what you’re doing. God has no observables. It’s an empty set. There’s nothing to believe but invention.
You’re right, there are angry atheists. Mostly they are angry about the long bloody history of religious people killing heretics; a history passed over in silence or with appeals for nuance by most religious people, and denied outright by the practitioners of religions that have not gone through the Enlightenment and who regret none of it, most notably those of Islam.
But angry atheists, no matter how personally objectionable, do not lend substance to an empty set nor justify belief in it.
Thanks for being civil, Ian. It’s appreciated.
No one is asking anyone to prove a negative. Prove a positive. Prove that life elsewhere exists and has the ability to travel in space.
Without the ability to travel to our closest star, I see little point in searching for basic life in the Universe. If we find ambiguous signs, it isn’t like we can send a mission there and find out. We will just end up with endless debates posited a endless hypotheses that no one can ever prove (but they might disprove some). The closest stars (less than 10 light years away) will require 100 years to reach and return, and there are not so many of those. The chances of finding life go up exponentiation once you go further out, but so does the amount of time to go there and establish any contact long past the point of being practical.
Looking for signs of intelligence is more useful as it at least has the potential to be unambiguous and informative in a singe lifetime. It will likely have to be performed by machines that self-repair – not exactly send human ambassadors to establish first-contact. It will not happen in my live time, not in the lifetimes of many generations after me. (unless there is a big breakthrough in physics – so let;s put our energy in hard physics and dump all the soft-science crap like climate science).
Until we master the ability to go there and look within a single lifetime, the game is pointless – although probably addictive to those that play it. The odds that we will find intelligent life forms (technologically advanced) are dimming ever year and now approaching zero.
The Drake equation is:
N= R x f(plnets) x N(potential life supporting) x F(live) x F(intelligence) x F(Civilization transmit) x L (time transmitting)
We are moving in the first four variables, but it is the last three that matter, Fi gets put out to be ONE because the only example we have is Earth. (why not include Mars, Venus, and the Moon, except that that knocks down the odds)
It is all guess work, and until we actually intercept a radio signal or have a verifiable visit, it will remain the stuff of SF.
Actually Pat, your civility to me is downright heroic. Let me tell you, you go around telling educated people that you don’t accept Evolution, and the first thing they do is seat you in the Stupid section.
Yeah I know, religion is mostly nonsense, and stupid nonsense at that. My tendency to just bring up questions of God is mostly mischief. I like to annoy people for fun. Not a nice thing to do, I know.
I do suspect that people, like Richard Dawkins, who are essentially proselytizers of Evolution really do understand the arguments from the other side. Evolutionists have a lot of little cheats going on to counter reasonable arguments to it. The first, and dirtiest, is they tell you that you’re too stupid to understand the theory, because you’re the kind of simpleton who believes the story of Noah’s Ark. You have to get them on that one right away. And when you say, the world is far too complex to have arisen by chance, they say, it took billions of years, and in that time many things are possible. Well no. Species evolve in real time, and are delimited by time. There is no gradual way to make an ostrich. An animal is either an ostrich or it isn’t, and the genesis of the first ostrich had to happen during the gestation period of an ostrich. There ain’t no million years about it.
I have never understood the logic of the proposition that Evolution is critical to the study of Biology. Come on. You don’t have to accept Evolution to understand how kidneys work, or trees grow, or how birds migrate.
Anyway, It was good of you to respond to my posts, Pat. Not everybody is willing to argue and be patient about it.
What finding? It is only a hypothesis!!