Aging stars that become red giants increase their luminosity and can wreak havoc on planets that were once in the star’s habitable zones. When the Sun becomes a red giant and expands, its habitable zone will move further outward, meaning Earth will likely lose its atmosphere, its water, and its life. But for planets further out, their time in the habitable zone will just begin.
Is there enough time for life to arise on these newly habitable planets?
While there are many ways to define habitability, most researchers base it on Earth-like conditions. A planet receiving the right amount of energy from its star can host liquid water if it has the right atmospheric conditions. Earth’s habitability has endured for billions of years as the Sun spends time in the main sequence.
But stars age. Main sequence stars begin to run out of fuel and enter the giant branch. Their luminosity grows, warming planets and moons that were previously frozen and allowing liquid water to persist on their surfaces. This creates a period of habitability. As the star’s luminosity continues to increase, these planets would then be too warm, ending their habitability. But things don’t stay that way for long. Eventually, the star enters the horizontal branch, and its luminosity decreases again. During this period of time, the habitable zone shifts inward again, and these previously frozen outer worlds can enter a second period of habitability.
New research looks at these shifting habitable zones and the planets and moons caught up in them. Its title is “Multiple Habitable Phases on Outer Exosolar Worlds.” It’s published in The Astrophysical Journal, and the lead author is Viktor Sparrman, a theoretical astrophysicist from the Department of Physics and Astronomy at Uppsala University, Sweden.
“The outer worlds’ time with habitable surface climates is key in evaluating the possibility of extraterrestrial life arising,” the authors write. As part of understanding these periods of habitability, the authors employ the term Time in Habitable Zone (TIHZ). “The times inside the habitable zone (TIHZ) are calculated for outer worlds orbiting between 5 and 45 au around a Sun-like star.”
Using our Solar System as an example, the authors show how increased insolation can alter the habitable zone as the Sun moves along its evolutionary track. As the Inner Habitable Zone and the Outer Habitable Zone change, planets enter and exit them.
The authors then compare the TIHZ to the estimated time it took for life to appear on Earth. Can some of these outer worlds have long enough TIHZ for life to appear? Yes, according to this research. “Multiple habitable phases are found for each outer world,” the paper states.
The researchers calculated both optimistic and conservative times for the Solar System planets in the habitable zone. Some of the planets pass through the habitable zones several times.
The researchers found that some outer worlds have secondary habitable phases that exceed the length of the primary phase. But that’s only if the water doesn’t disappear during the time between the two phases. It’s possible that runaway greenhouse conditions could eliminate enough water to negate habitability in the secondary phase.
But the team’s results showed that it’s highly unlikely that worlds as distant as Saturn is from the Sun would lose enough water to eliminate habitability. “At the orbiting distance of Saturn, none of the hypothetical outer worlds lose as much water as there is in Earth’s oceans using either the PARSEC or Dartmouth solar evolution models,” the authors explain.
The research shows that both individual TIHZs and total TIHZs for planets diminish as orbital distance increases. But in our Solar System, the total surface area and water budget for the outer planets exceeds that of the inner planets. What does that mean for habitability?
“If life originated in shallow water on the surface of Earth, then a larger surface area may increase the probability that life arises, depending upon surface water inventory,” the researchers explain in their paper. “Life originating in shallow waters is supported by UV radiation being an energy source able to supply sufficiently high activation energy for starting the prebiotic reaction processes.”
But here’s the burning question: is there enough time for life to emerge on these outer planets as they spend time in the habitable zones? There are two parts to that, according to the authors. One is the amount of time for a planet to become habitable, which in simple terms means to develop surface water, and the other is how long it takes life to appear.
The only way to figure that out is to compare it to our best estimates of how long it took life to appear on Earth. “Currently, the oldest fossil records are stromatolites, dating back 3.5–3.8 Gyr ago,” the authors write. “Using this conservative definition, the time for life to arise on Earth is then ?1 Gyr since Earth’s age is 4.5 Gyr.” Life may have appeared sooner than that, but the evidence didn’t survive the planet’s tectonic recycling.
Unfortunately, the results show that the TIHZs for the outer planets are less than the upper boundary for the amount of time we think it took for life to develop on Earth. “Pessimistically, if one assumes this upper bound as the limit, it is unlikely that life forms on any one of these outer worlds,” the authors explain.
But there are multiple outer worlds. Each one represents a separate opportunity for life to emerge. There are even more moons. How does this affect the outcomes? This is where things get even murkier.
“The HZ is a construct which should be interpreted as a zone where the probability for life to exist is heightened,” the researchers write. Any worlds that are fortunate enough to find themselves near the HZ’s center are more Earth-like and more likely to give rise to life. “Whether worlds at the edges of the HZ would be more or less likely to host life is difficult to quantify,” they explain.
It’s certainly plausible that life could arise on these outer worlds at the edges of habitable zones. The main problem is that we don’t know how long it took for life to arise here on Earth. And since the outer planets are also larger, there’s more available surface area and more opportunity for the spark of life to activate.
But what if life is travelling around on comets, asteroids—even dust—and is constantly spreading via panspermia? Would that shorten the time? The authors don’t tackle that topic; it’s largely conjecture.
“Considering the multitude of outer worlds, the possibility that life forms on any one of them during any of their TIHZs is sufficient to warrant consideration in the search for extraterrestrial life,” the authors conclude.