Finding life beyond our planet would be an extraordinary discovery. Perhaps it might even be the singularly most important moment in all of history. There are many ways we might be able to suss out extraterrestrial life. The non-profit research institute SETI (short for Search for Extraterrestrial Intelligence), is one of those on the job. Astrophysicists are also using telescopes to find planets that could be suitable for life. The problem is, the search might be hampered by the fact that we tend to focus on planets we deem to be Earth-like. Could this be the wrong approach?
What Makes a Planet Habitable in the First Place?
There are many factors that go into deciding whether a planet is considered habitable. To summarize, they range from density, temperature, composition, chemical makeup, and whether the planet is in what scientists call the Goldilocks-zone (or the habitable zone). This means the exoplanet orbits its star from a distance that isn't too close or too far away-leaving the potential for liquid water to flow on the planet's surface.
All of these are things that determine whether newly discovered exoplanets are deemed Earth-like or habitable. However, in a recent study, a group of astronomers examined whether we should be looking at planets (or even moons) that aren't necessarily Earth-like, but more conducive to different kinds of life. These planets might be considered not just habitable, but superhabitable-even more so habitable than Earth itself.
What is a Superhabitable Planet?
Simply put, a superhabitable planet is one in which conditions are more suitable not just for the emergence of life, but for the sustainability of it. In order to identify a superhabitable planet, we must also look beyond any geo or anthropocentric lens, and instead, look through a biocentric lens.
There are a few general characteristics of a superhabitable planet:
- A superhabitable exoplanet would orbit a K-type star. These are defined as main-sequence stars with less mass than our Sun, but more long-term stability. For example, stars like the Sun typically remain stable for approximately 10 billion years. Then they begin to run out of fuel and swell into red giants, before ultimately becoming white dwarfs. K-type stars, on the other hand, can "live" for 18 to 34 billion years. That's quite a difference, not just in terms of age but in radiation output. Ultimately, any habitable planet circling one of these stars would have more time for life not just to develop, but to evolve and thrive.
- A superhabitable exoplanet would likely be approximately 1.5-1.6 Earth masses. This larger volume could allow for a more complex terrain than Earth, and a larger area of surface water. This also increases the odds that the planet will have plate tectonics, an active core, a stable mantle, and, because of its stronger gravitational pull, it's expected to have a denser atmosphere. Importantly, the planet must be able to retain a lot of the gases leftover from the planet's formation. Otherwise, it might not have liquid water, suitable levels of oxygen, and too cool surface temperatures from the lack of greenhouse gases. The atmosphere also protects the surface of the planet from harsh radiation, and of course, space debris.
- If the planet were to accumulate an adequate reservoir of water during its formation and in subsequent years (say, icy asteroids and comets crashed into the planet and deposited water), its ocean depth must also be taken into consideration. Any organisms forming beneath the surface need a source of energy — this could be heat and light from the surface, or energy from subsea vents. However, in general, the deeper the ocean, the less energy they receive. Additionally, research shows that warmer, wetter environments, for example, rainforests, tend to be more conducive for life.
- Superhabitable planets would be in the appropriate age range. It takes many billions of years for organic matter to transform into complex organisms, and for those complex multicellular organisms to then evolve into an intelligent species. Earth is currently 4.6 billion years old. The first lifeforms may have appeared 3.5 billion years ago. The ideal range for a superhabitable planet is thought to be approximately 5 to 8 billion years old. The planet would have had time to experience calamity, such as asteroid impacts, ice ages, and other defining events, and for life to evolve in response. The planet must not be too old though, as older planets are more likely to have weaker internal heat sources and weak magnetospheres.
Have We Discovered Any Superhabitable Planets?
In 2020. astronomers created a shortlist of 24 planets they thought could be considered superhabitable candidates, out of the more than 4,000 known exoplanets. These planets were specifically identified by first looking at planetary systems that have terrestrial planets in the habitable zone, and which orbit K-type main-sequence stars. Some of these planets were 1.5 times the mass of Earth and estimated to be between 5 and 8 billion years old, with similar temperatures.
Only one of the candidates — KOI 5715.01 — fits almost all of the criteria. The only problem is, models predict it might be cooler than Earth, which could definitely be a hindrance for the evolution of life. Moreover, all of the planets identified are beyond 100 light-years of Earth — making it more difficult to study them in-depth using NASA's TESS mission. However, some are within the 700 light-year range, which is basically a hop and skip away in celestial terms.
“With the next space telescopes coming up, we will get more information, so it is important to select some targets,” said Schulze-Makuch, a professor with WSU and the Technical University in Berlin. “We have to focus on certain planets that have the most promising conditions for complex life. However, we have to be careful to not get stuck looking for a second Earth because there could be planets that might be more suitable for life than ours.”
One caveat, and something that is important to note, is that calling a planet superhabitable doesn't mean it can't be a barren wasteland, completely devoid of all the ingredients needed for life. Our only reference point is the biodiversity of Earth. We've seen that life can find a way in the most unlikely of places, from the highest mountains to the hydrothermal vents on the ocean floor. However, we neither have either the full picture nor all of the parameters needed to identify exactly what a planet needs for life to begin.
We might find that many of the exoplanets we've already cataloged qualify as superhabitable planets once we take a closer glance — or none of them may qualify. Astronomers may eventually find some within a couple light-years of Earth, which makes this research very important in terms of finding a truly habitable planet.