In our quest for life beyond the Solar System, it makes sense to look for a world like our own.
We’ve long hoped to find an Earth-sized world around a Sun-like star at the right distance for liquid water as our first step, and with thousands of planets in our coffers already, we’re extremely close.
But not every world with the right physical properties is going to have life; we need additional information to know whether a potentially habitable world is actually inhabited.
The follow-up would be to analyze the planet’s atmosphere for Earth-like signatures: potential signs of life.
Earth’s combination of atmospheric gases — nitrogen, oxygen, water vapor, carbon dioxide and more — has been assumed to be a dead giveaway for a planet with life on it.
But a new study by planetary scientist Dr. Sarah Hörst’s team throws that into doubt. Even worlds rich in oxygen might not harbor aliens, but an impostor process that could fool us all.
The scientific story of how to even reach that point is fascinating, and closer to becoming a reality than ever before.
We can understand how this happens by imagining we were aliens, looking at our Sun from a large distance away, trying to determine if it possessed an inhabited world.
By measuring the slight variations in the frequency of the Sun’s light over long periods of time, we’d be able to deduce the gravitational influence of the planets on them.
This detection method is known either the radial velocity or the stellar wobble method, and can tell us information about a planet’s mass and orbital period.
Most of the early (pre-Kepler) exoplanets were discovered with this technique, and it’s still the best method we have for both determining planetary masses and confirming the existence of candidate exoplanets.
We also need to know the size of the planet. With the stellar wobble alone, we’ll only know what the mass of the world is relative to the angle-of-inclination of its orbit.
A world that’s the mass of Earth could be well-suited to life if it’s got an Earth-like atmosphere, but it could be disastrous for life if it’s an iron-like world with no atmosphere at all, or a low-density, puffy world with a large gaseous envelope.
Most of them orbit red dwarf stars — the most common class of star in the Universe — which means the forces should tidally lock them: the same side should always face the star. These stars flare often, posing a danger to any potential atmospheres on these worlds.
Historically, when we’ve looked to the skies for evidence of life beyond Earth, we’ve been biased by hope and what we know on Earth.
Theories of dinosaurs on Venus or canals on Mars still linger in our memories, and we must be careful that extraterrestial oxygen signatures don’t lead us to falsely optimistic conclusions.
We now know that both abiotic processes and life-dependent ones can create an oxygen-rich atmosphere.
The hard problem, then, will be disentangling the potential causes when we actually find our first oxygen-rich, Earth-like exoplanet.
Our reward, if we’re successful, will be the knowledge of whether or not we’ve actually found life around another star.
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