The search for life involves the most sophisticated observational machines known to humanity.
They peer out across the light-years, looking for some proof – any proof – that other life exists, out there.
What if, despite all our efforts, those observations turn up NO evidence of life elsewhere in our Milky Way Galaxy?
That’s a scary question. What if we continue building more and more sensitive telescopes to survey temperature exoplanets and still find nothing?
How many planets do we need to study to come to the conclusion there’s nobody but us in the cosmos?
At present, astronomers know of only a tiny fraction of worlds, around 7,000 at last count.
A team of researchers led by Dr. Daniel Angerhausen of ETH Zurich and the SETI Institute contemplated what we might learn about the possibilities of life in the Universe if future searches turn up as empty as current ones are doing.
They tackled those questions via a Bayesian analysis to incorporate ever-changing information to compute and update probabilities in the number of exoplanets that could have life.
In their work, the team members assumed that the number of planets observed would be large enough to draw strong conclusions about the prevalence of habitability and life in our galactic neighborhood.
However, there are still uncertainties. Even with advanced instruments, exoplanet searches will need to carefully account for uncertainties, unknowns, and biases (such as assumptions made about certain types of worlds).
Angerhausen’s team established the minimum number of exoplanets scientists need to figure out how many worlds could exist with life. If scientists examined 40 to 80 exoplanets and end up NOT detecting life on any of them, it would imply that fewer than 10 to 20 percent of similar planets harbor life.
Extrapolate across the Milky Way, and you will discover that only about 10 billion of its worlds could be inhabited planets. For our part of the galaxy, it might be a very small number indeed.
Challenges in exploring distant worlds
Studying planets around other stars is not an easy task. First, you have to “suss them out” from the bright light of their stars.
Then, once you find them, you need to make some assumptions about their makeup. Are they rocky with atmospheres (like Earth)? Are they gas giants? Lava worlds? Ice giants? How close are they to their star? Do they have moons? If they do have a life-sustaining environment, what kind of life? And so on.
Each type of world determines whether or not life can exist there. On top of that, every observation has its limitations, and that is reflected in the data. For example, observations may miss certain atmospheric clues that indicate life (or its non-existence). Scientists may completely skip over certain planets and dismiss them as uninhabitable when they may be perfect for life. Or, they might miss a set of planets altogether due to observational limitations. The uncertainties are daunting.
“It’s not just about how many planets we observe – it’s about asking the right questions and how confident we can be in seeing or not seeing what we’re searching for,” said Angerhausen. “If we’re not careful and are overconfident in our abilities to identify life, even a large survey could lead to misleading results.”
Taking a lot of uncertainty out of the search for life
Future searches for life on other worlds are on the drawing boards. Their success will depend on scientists making the right assumptions about where life can (and can’t) exist.
Those assumptions – based on prior knowledge and ongoing observations – should reduce a great many uncertainties about habitable worlds.
For example, there’s the Large Interferometer for Exoplanets (LIFE) led by ETH Zurich and proposed for launch in the next decade or so. It’s designed to search for life on exoplanets and, at the same time, catalog the diversity of worlds as part of its observations.
To do that, the onboard instruments will measure the atmospheric composition of temperate terrestrial-type planets in the galaxy. The spectrometer will study the atmospheric gases for traces of life-relevant molecules.
Think of it this way: if such a probe were to study Earth’s atmosphere, it would detect oxygen molecules and such gases as methane and other hydrocarbons as indicative of life.
To avoid null results, LIFE’s searches will depend on the Bayesian statistics Angerhausen and colleagues have done. Essentially, it will study the atmospheres of dozens and dozens of worlds that are similar to Earth in mass, radius, and temperature.
The goal is to find the signatures of water, hydrocarbons related to life, and other biosignatures.
The results should “amp up” the numbers of exoplanets studied to give a more statistically accurate accounting of planets that could (or do) support life. In particular, the science teams want to see how many are in our neck of the galactic neighborhood.
We’ll always have some uncertainty
Ultimately, LIFE’s observations (and other surveys) will help answer a whole new set of questions about life-bearing worlds. How many planets have life?
Which fraction of them are rocky worlds and what percentage of those are in habitable zones?
Of those, which ones have clear signatures of water vapor, oxygen, and methane in their atmospheres? The work that Angerhausen’s team is doing to compute statistical probabilities will help the LIFE mission (and other similar missions) ask the right research questions when it comes to the search for habitable worlds.
If the result is a single positive detection, according to Angerhausen, then uncertainties become reality. “Even if we don’t find life,” he pointed out, “We’ll be able to quantify how rare – or common – planets with detectable biosignatures really might be.”
Even if future surveys such as LIFE find no evidence of life on neighboring exoplanets, they will still open a window to understanding just how rare or common habitable places are in the Universe.
By carefully considering uncertainties and asking precise questions, scientists can create powerful tools for understanding our place in the cosmos. Ultimately, exoplanetary science isn’t just about finding answers to questions about habitability and the existence of life.
It’s also about asking the right questions and embracing uncertainty as part of the journey.
Written by Carolyn Collins Petersen/Universe Today.
