When the Sun becomes a red giant, will the outer planets and moons become more temperate and conducive to Earth-like life?
Terrence A. Murray
Cincinnati, Ohio
When the Sun runs out of hydrogen fuel and expands into a red giant, it will eventually encompass the innermost planets of the solar system, out to about Earth’s orbit. Being closer to our enlarged central star will mean drastically hotter temperatures in the planets that remain in the outer solar system, as well as their moons.
Alan Stern, a researcher at the Southwest Research Institute (SwRI) and expert on the Kuiper Belt, writes in a 2004 Astrobiology article that during our Sun’s red giant phase, the habitable zone where liquid water can exist will shift out to some 10 to 50 AU. So, more temperate? Certainly. But let’s explore the trickier part of your question: Will those regions become more conducive to life?
The four outer planets don’t have any solid surface to speak of, and do have enormously deep atmospheres made largely of the light elements hydrogen and helium. Traditionally, astronomers thought planets like these would be blown apart if they orbited close to their stars, but we now know of quite a few examples of hot Jupiters — gas giants tightly orbiting their stars, albeit with extended atmospheres, much like a puffed-up marshmallow. So it’s possible that these planets might remain intact as the Sun expands, but they won’t become any better prospects for life.
The answer is more optimistic for some of their moons, where astronomers have long searched for clues to habitability. Moons including Enceladus, Europa, Titan, and Triton all probably have oceans of water under their surface. And that’s not to speak of all the dwarf planets in the Kuiper Belt, including Pluto, many of which have at least some water currently locked up in the form of ice, either on or below the surface. Rich organic materials, another requirement for life, also appear over and over again in the outer solar system.
Astronomers speculate even now about the possibility of life in the subsurface oceans we know about. If more worlds became warm and wet with an expanded habitable zone, the options would only increase. Stern also points out this “delayed gratification habitable zone,” as he calls it, would suffer fewer collisions and less harmful solar radiation than the traditional (current) habitable zone, which could increase chances for life.
On the other hand, we still don’t know how life came about on Earth to begin with. And because the habitable zone shifts even during the red giant phase as the Sun continues to evolve, no planet in our solar system will spend more than half a billion years in the new habitable zone — and it took just about that long for life to develop on Earth. Can our solar system do it again? We’ll find out … in about 5 billion years.
Korey Haynes
Contributing Editor
