Red dwarf stars, or M dwarfs, have recently been hailed as the best places to discover alien life. They are by far the most common stars in our galaxy, making up 75 percent of all stars. They are also the longest-lived-they can burn for trillions of years, far longer than the ten-billion-years lifespan of our Sun. What's more, nearly all of them may have a planet in the habitable zone.

But new research now adds important nuances to their story. In a new model, Lisa Kaltenegger and Ramses M. Ramirez at Cornell University in Ithaca, New York, took into account the habitable zone of younger red dwarf stars.

The habitable zone is the region around a star where water might remain liquid on the surface of a planet. For young stars, that region is located further away since they are bigger and brighter than full-grown stars.

"Our study adds new targets to the search for potential habitable worlds," Kaltenegger says. "So far no one has thought to look for these infant Earths."

But the study also calls into question the habitability of many previous candidates-namely, all the planets that are currently located in the habitable zone of the full-grown stars. These planets would spend too much time outside of the habitable zone during their formative years with, their water boiling away, and along with it their chance at life.

The paper will be published in the Jan. 1, 2015, issue of Astrophysical Journal Letters.

Young Hot Stars
New stars form when clouds of dust and gas collapse under their own gravity. As a large disk begins to swirl, a protostar begins to heat up and glow at the center, while surrounding matter clumps together to forms baby planets.

The long and stable adult phase of a star, or the so-called main sequence, begins when the star starts to fuse hydrogen atoms. The pre-main-sequence, or "teenage" years of a star, takes place when the star has acquired all of its mass but has not yet begun to burn hydrogen.

"Generally, we don't think of these young stars as interesting because for our Sun that period is relatively short," Kaltenegger says. "But red dwarfs can spend up to 2.5 billion years in the pre-main-sequence, potentially providing habitable conditions for nearly as long. On Earth, life appeared only within the first billion years."

These younger stars are actually larger and brighter than full-grown stars of the same mass. They are still shrinking under their own gravity, and this gravitational contraction heats up their core. For this reason, the habitable zone is wider and farther out.

"This opens interesting new possibilities," Kaltenegger says, "because these planets further out are easier to study. The next generation of telescopes, such as TESS, PLATO, ELT, and JWST, should have good enough resolutions to detect these planets and study their atmosphere."

As the star enters its adult phase and the habitable zone migrates inward, whatever life might have developed there could then move underground or underwater, the authors speculate.

Growing Up in the Dead Zone
For planets in the habitable zone of the full-grown stars, that's a different story. These planets would likely loose their water early on, as they would be on the inward side of the young star's habitable zone. These worlds would essentially spend their formative years with runaway greenhouse effects, much like Venus, and all water would boil away, leaving little chance for life.

Unless, perhaps, these planets are rehydrated later on. "Our own planet gained additional water after this early runaway phase from a late, heavy bombardment of water-rich asteroids," says Ramirez. "Planets at a distance corresponding to modern Earth or Venus orbiting these cool stars could be similarly replenished later on."

Another recent study, by Rodrigo Luger and Rory Barnes of the University of Washington, came to the same conclusion, though they put a stronger emphasis on the loss of water. Their paper will soon be published in the journal Astrobiology.

"The habitability of many planets around M dwarfs must be questioned," the authors wrote. "While next-generation space telescopes such as JWST may be capable of detecting certain biosignatures in these planets' atmospheres, such observations will be extremely costly and require extensive amounts of valuable telescope time," they added.

"Knowing in advance which planets are viable candidates for hosting life is therefore crucial, since it is possible that many planets in the habitable zone are not actually habitable for life as we know it."

"Mirage Earths"
Luger and Barnes also highlighted another important implication. In addition to loosing their water early on, planets in the habitable zone of the full-grown stars may end up with large amounts of oxygen in their atmosphere.

That oxygen, however, would not mean that there is life. Red dwarf stars emit a lot of radiation, which would split the water molecules into hydrogen and oxygen atoms. The lighter hydrogen atoms would be lost to space more easily, while the heavier oxygen atoms would remain behind.

In this case, oxygen would, in fact, be an anti-biosignature, and produce what the authors called "mirage Earths."

In the end, though, our knowledge about life and habitable worlds is still limited to one example-our own.

As Kaltenegger puts it: "In the search for planets like ours out there, we are certainly in for surprises. That's what makes this search so exciting."