Here's Why Alien Life, Habitable Worlds Might Be Found Near 'Goldilocks' Stars

The search for life and habitable worlds have been two of the major goals for many ongoing and upcoming space missions. Some scientists have begun narrowing the search for stars that are most likely to host planets with the necessary ingredients to support life.

According to a new study published in The Astrophysical Journal, K stars might be a good place to start looking for signs of life. These stars are dimmer than the Sun but brighter than the universe's faintest stars, M stars or "red dwarfs."

As for why scientist and study author Giada Arney believes K stars are promising targets, this class of stars apparently has a very long life span compared to the Sun, which is about halfway done with its 10-billion-year life. Since K stars could live between 17 billion to 70 billion years, there is also a lot of time for life to evolve in the planets surrounding it. K stars are also less volatile in their youth than M stars.

"I like to think that K stars are in a 'sweet spot' between Sun-analog stars and M stars," Arney, who is a research scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, said in a statement.

It is possible that M stars, the most common star type in the galaxy, might also host some habitable planets. These stars comprise 75 percent of the universe and could live up to a trillion years since they use less fuel. M star TRAPPIST-1 has also been found to have seven rocky planets around the same size as Earth orbiting around it.

However, Arney suggested that K stars are more likely to host habitable worlds over M stars because the latter experiences more extreme activity during their youth, releasing powerful stellar flares more often than young Sun-like stars.

M stars would also make liquid water impossible to find even in planets orbiting in its habitable zone because they produce so much energy for up to a billion years after their birth, enough to boil off oceans. And without water, one of the necessities for life, there is very little chance of life evolving in these planets.

The presence of both oxygen and methane in a planet's atmosphere is considered to be a major sign of life or biosignature. These two gases react with each other, destroying each other, so their presence implies that something, possibly a life form, is producing them both at a fast rate.

In order for exoplanets to be seen by our telescopes, they would need to have a lot of oxygen and methane in their atmosphere. And in the study, Arney wrote that an oxygen and methane biosignature is likely to be stronger around a K star than a star like our Sun.

"When you put the planet around a K star, the oxygen does not destroy the methane as rapidly, so more of it can build up in the atmosphere," said Arney. "This is because the K star's ultraviolet light does not generate highly reactive oxygen gases that destroy methane as readily as a Sun-like star."

Arney also pointed out that it would be easier to spot and observe exoplanets orbiting K stars than those around Sun-like stars since the former star type is dimmer.

The research scientist noted that nearby K stars like 61 Cyg A/B, Epsilon Indi, Groombridge 1618, and HD 156026 would be a good place to start searching for biosignatures.