Habitable Planets in the Same System

Learning that our own Solar System has a configuration that is only one of many possible in the universe leads to a certain intellectual exhilaration. We can, for example, begin to ponder the problems of space travel and even interstellar missions within a new context. Are there planetary configurations that would produce a more serious incentive for interplanetary travel than others? What would happen if there were not one but two habitable planets in the same system, or perhaps orbiting different stars of a close binary pair like Centauri A and B?

My guess is that having a clearly habitable world — one whose continents could be made out through ground-based telescopes, and whose vegetation patterns would be obvious — as a near neighbor would produce a culture anxious to master spaceflight. Imagine the funding for manned interplanetary missions if we had a second green and blue world that was as reachable as Mars, one that obviously possessed life and perhaps even a civilization.

Solar systems with multiple habitable planets are the subject of an interesting new paper from Jason Steffen (UNLV) and Gongjie Li (Harvard-Smithsonian Center for Astrophysics), who point to the fact that the Kepler mission has found planet pairs on similar orbits, with orbital distances that in some cases differ by little more than 10 percent. Mars is at best 200 times as far away as the Moon, but as Steffen notes, multi-habitable systems will produce much closer destinations:

“It’s pretty intriguing to imagine a system where you have two Earth-like planets orbiting right next to each other,” says Steffen. “If some of these systems we’ve seen with Kepler were scaled up to the size of the Earth’s orbit, then the two planets would only be one-tenth of one astronomical unit apart at their closest approach. That’s only 40 times the distance to the Moon.”

We can’t know at this point whether any of the Kepler candidates have life or not, but consider this: Kepler-36, about 1530 light years away in the constellation Cygnus, is known to be orbited by a ‘super-Earth’ and a ‘mini-Neptune’ in orbits that differ by 0.013 AU. The outer planet orbits close enough to the inner that we can pick up obvious transit timing variations (TTV) for both. Locked in a 7:6 orbital resonance, these worlds are representative of this kind of planetary configuration. Imagine the changing celestial display from the surface of the super-Earth as the larger planet sweeps out its orbit. Now consider the same view if both planets were clearly habitable!

Image: A gas giant planet spanning three times more sky than the Moon as seen from the Earth looms over the molten landscape of Kepler-36b. Credit: Harvard-Smithsonian Center for Astrophysics.

Steffen and Li focus in on two key issues. Specifically, what sort of variations in obliquity might be caused by such close planetary orbits? Obliquity measures the angle between the planet’s spin and its orbital axis — Earth’s 23.5° tilt relative to its orbit explains seasonal change and has obvious effects on climate. So we’d like to know whether close orbits disrupt planetary climates enough to wreak climatic havoc. The answer in most cases is no. From the paper:

We found that obliquity variations are generally not affected by the close proximity of the planets in a multihabitable system. Also, obliquity variations of close pairs embedded in the solar system or of potentially habitable planets in a system with a close pair were not sufficient to significantly reduce the probability of having a stable climate. Only in cases where the inclination modal frequencies coincide with the planetary precession frequency did large obliquity variation arise.

With planets this close, however, an even larger question is whether life on one planet can influence the other. We have abundant evidence of rocks from Mars that have fallen to Earth, leading to the possibility that other planets could have delivered life-bearing materials here in the process called lithopanspermia. Much closer planets should be far more susceptible to this process. Clearly, the possibility exists for life branching out from the same roots, taking different evolutionary paths just as occurs on remote islands here on Earth.

The simulations used by Steffen and Li demonstrate that two Earth-like planets in orbits like those found around Kepler-36 would have a much greater opportunity for exchanging materials than planets do in our Solar System. The relative velocities of the planets — and thus the velocities of the ejected particles — would be much less than in the case of a transfer of materials from Mars to the Earth. Biological materials transferred by collision ejecta have been considered on individual planets (with material falling back onto other parts of the same world), between binary planets or habitable moons, and even between different star systems.

Steffen and Li’s simulations show that the nearer the planets are to each other, the higher the success rate of ejecta transfer. Moving biological materials between two worlds becomes almost as feasible as moving them from one place on a single planet to another region of that planet, and the energy of the impact needed to make the transfer is much less than in our Solar System, leading to higher survivability. We can add in the fact that the time needed for biological materials to survive an interplanetary journey is that much shorter.

And there is this final factor:

…we found that the smaller the velocity of the ejected material the more uniformly they can be sourced across the originating planet. With high velocity ejecta, the range of initial longitudes is constrained relative to the direction of motion.

The result: Debris from a single impact is far more likely to strike the destination planet at multiple locations in rapid succession than when planets are spaced farther apart. That too increases the chance of life catching hold. Processes like these, which could also occur around planets with large habitable moons, allow life to spread readily within the same system. The paper adds:

Not only will panspermia be more common in a multihabitable system than in the solar system, but the close proximity of the planets to each other within the habitable zone of the host star allows for a real possibility of the planets having regions of similar climate—perhaps allowing the microbiological family tree to extend across the system. There are many things to consider in multihabitable systems, especially in the cases where intelligent life emerges.

In this UNLV news release, Steffen follows up that last thought:

“You can imagine that if civilizations did arise on both planets, they could communicate with each other for hundreds of years before they ever met face-to-face. It’s certainly food for thought.”

The scenario is striking, and I’m hoping readers can come up with some science fiction titles in which multiple habitable planets in the same system are the background for the tale.

The paper is Steffen and Li, “Dynamical considerations for life in multihabitable planetary systems,” accepted at The Astrophysical Journal (preprint).