Red Dwarf Stars Probably Not Friendly for Earth 2.0
The stellar runts of the galaxy probably aren't so great for nurturing Earth-like worlds, say scientists running new simulations of the formation of planets around a variety of stars.
Red dwarf stars - the stellar runts of the galaxy - probably aren't so great for nurturing Earth-like worlds, say scientists running new simulations of the formation of planets around a variety of stars.
Astronomers have discovered a huge variety of alien worlds orbiting all types of stars, but one type of star, the M dwarf, has stuck out as one location where Earth-like exoplanets could be nurtured. Red dwarfs are plentiful in our galaxy and many nearby red dwarfs are known to play host to small and (likely) rocky exoplanets. Red dwarfs are long-lived and should there be a suitable rocky world orbiting within the star system's habitable zone, surely there's a good chance for life to evolve?
Unfortunately, there's some limitations with this thinking. As the habitable zone around M dwarfs is extremely compact (these stars are smaller and therefore dimmer than our sun, for example), any potentially habitable world would need to orbit a red dwarf very closely. In these cases, the rocky world will likely become "tidally locked" with the star, forcing one hemisphere to endure an eternal day, while the other freezes in an eternal night - certainly not very "Earth-like" in the classical sense.
Also, many of these dwarfs are thought to be tumultuous little stars, erupting with powerful flares that could extinguish any form of biology (as we know it) before it can get a foothold. Already red dwarfs are looking a little unlikely as hosts for bona fide Earth-like worlds.
But as the vast majority of stars in our galaxy are red dwarfs, surely, just because there are so many, a few of these red dwarf star systems have formed with the right balance of planetary material and perfectly-located orbits that worlds similar to Earth have been able to coalesce?
According to new research from Tokyo Institute of Technology and Tsinghua University, not so much.
The researchers, Shigeru Ida (Tokyo Tech) and Feng Tian (Tsinghua), first defined what "Earth-like" means. For an alien world to meet this classification, it needs to be of the approximate mass and physical size as Earth. It also needs to have a similar water:land mass ratio. Exoplanets with too little water are considered "dune worlds" - with too little water for Earth-like biology to take hold - and those with too much water are known as "water worlds" - with chaotic climates and poor nutrient supply.
Ida and Tian found that, for an Earth-like exoplanet, with just the right water:land ratio to form around an M dwarf, there needs to be an extremely unlikely balance of planetary material very early in that world's formative years.
As red dwarfs form pre-main sequence, they are extremely luminous, blasting out vast quantities of energy. They then quickly settle down into a more quiescent, cooler state. For any would-be Earth-like exoplanet forming in a habitable orbit, the early years of a red dwarf's life are hard. Due to their close proximity, these worlds would be cooked, with extreme implications for their future habitability.
After running their simulation for 1000 stars 30 percent the mass of our sun (the approximate mass of M dwarf stars), 5,000 exoplanets with Earth-like masses formed. However, only 55 of those planets formed within the stars' habitable zones and only 1 formed with the "perfect" Earth-like water:land ratio. 31 habitable zone planets turned into "dune worlds" and 23 turned into "water worlds."
So far, things are looking bleak for red dwarfs hosting Earth-like planets, but the story changes as the stars become more massive.
For example, for 1,000 stars 50 percent the mass of our sun, 75,000 exoplanets formed and 9,000 of them had Earth-like masses. 292 of those worlds formed inside the habitable zone and 12 were truly Earth-like.
But after simulating 1,000 sun-like stars, 38,000 exoplanets formed, 8,000 of them of Earth-mass. Although this may be a lower number, 407 formed inside the habitable zone and 271 of those formed with just the right water:land ratio, making sun-like stars the most likely place to form Earth-like exoplanets.
Although these are simulations, they take their lead from established planetary formation theories and will ultimately help guide future exoplanet-hunting missions. But things aren't looking good for red dwarfs being particularly good locations for Earth-like planets, and therefore life, to form.
"We suggest that stars close to the size of the sun should be the primary targets for detecting Earth-like planets," the researchers conclude.
Artist's impression of planets orbiting a red dwarf star. It's unlikely any of those worlds are habitable.
Cowboys & Aliens are Coming!
July 29, 2011 --
If aliens are going out of their way to kick up dust in the Wild West, as they do in the upcoming movie "Cowboys & Aliens," they must be coming from somewhere. Life could take root on a moon or a meteorite. But to nurture the kind of life that could destroy our saloons and harass our livestock, a planet might be the most suitable. So far, Kepler, a NASA orbiting telescope that searches for planets beyond our solar system, has detected over 1,200 exoplanets. Surely there must be a few candidates among this group that could meet some of the most basic requirements to host life? Explore some far-out worlds that could support aliens, be they cattle-rustling characters or a more peaceful people.
First, let's lay out some basic criteria. Kepler hasn't identified many rocky worlds and a solid surface is essential for life to take root. Size matters: The mass of the planet helps astrophysicists infer what it's made of. Some planets are Earth-sized. Others are several times the size of our planet. And then there are gas giants, which can range from "Neptune sized" to "super-Jupiters." Orbit: To support life, a planet must be in a stable orbit around its star -- no planets with wonky orbits that will eventually dump them into their star for a fiery death. Goldilocks Zone: This is a region not too hot or too cold that gives the planet enough distance from its parent star to have liquid water, key for life. Loner Stars: Single stars make better parents. In 2010, a pair of closely orbiting binary stars was spotted surrounded by what could be the debris of former planets. Unknowns: Some factors for life can't be confirmed one way or the other from the data available about extrasolar planets. These include: water, chemical compounds such as ammonia; a nitrogen-rich atmosphere; a magnetic field to repel solar and cosmic radiation; and more. BUT, some planets do have a head-start, beginning with Gliese 581D.
Located a mere 20 light-years away, practically our backyard in cosmic terms, Gliese 581d is situated on the "outer fringes" of the Goldilocks zone, orbiting a red dwarf star. The planet may be warm enough and wet enough to support life in much the same manner as Earth. It might also contain a thick carbon atmosphere. If we ever need a new Earth and have the means to get there, Gliese 581d may be our best bet for now.
When it was first detected and reported last year in Astrophysical Journal, Gliese 581g appeared to be the perfect candidate for a true "Earth-like" planet. Located in the same star system as Gliese 581d (and detected earlier), Gliese 581g seemed to be the right size and located within a habitable zone away from its parent star. Gliese 581g was said to have three times the mass of Earth, making it possible for the planet to hold an atmosphere. However, since its discovery, follow-up studies have alleged that Gliese 581g might have been a false alarm. In other words, the planet might not exist at all.
Dubbed a "waterworld" and located a mere 42 light-years from Earth, GJ 1214b orbits near a red dwarf star about one-fifth the size of our sun. What makes this planet unique is that it appears to be primarily composed of water, although GJ 1214b is 6.5 times the mass of Earth and 2.7 times wider, which classifies it as a "super-Earth." This planet also has a steamy atmosphere composed of thick, dense clouds of hydrogen, which, although it might not the case with this planet, could incubate life.
Situated 150 light-years from Earth, HD 209458b is a planet that holds traces of water vapor in its atmosphere, and also contains basic organic compounds that, on Earth, foster the development of life. But there are two factors working against HD 209458b as a suitable habitat. The planet is very hot due to its close proximity to its parents star, and it's a gas giant, so no solid surfaces.
If Kepler-10b were located further from its parent star, it might have had a chance of hosting life. Kepler-10b was the first "iron-clad proof of a rocky planet beyond our solar system" back in 2001. It was even dubbed the "missing link" of extrasolar planetary research. When it comes to the search for life, though, Kepler 10-b is missing a lot of other ingredients -- just minor things like water or an atmosphere.
When venturing to a new star system to explore the possibility of extraterrestrial life, trying a star that has already shown itself to nurture planets -- even if they're not the kind you're looking for -- could be a promising strategy. Project Icarus, an ambitious five-year study into launching an unmanned spacecraft to an interstellar destination, has identified two stars located within 15 light-years that might fit the bill: "epsilon Eridani, a single K star 10.5 light-years away, and the red dwarf GJ 674, 14.8 light-years away." Indirect evidence has also shown that epsilon Eridani may already hold smaller worlds scientists simply haven't detected yet. Also, red dwarf star systems generally may be a safe haven for life.
Are We Alone?
Taking into account the number of exoplanets that have been detected, as well as the vastly greater number that are estimated to be out there, some astrophysicists are convinced that extraterrestrial life is inevitable. After all, the Milky Way may be loaded with as many as 50 billion alien worlds. Some even think we'll find alien life by 2020. Others, however, say it may not exist at all. Recently, astrophysicists David Spiegel of Princeton University and Edwin Turner from the University of Tokyo suggested we might be alone in the universe, based on their interpretation of the Drake equation, a formula meant to determine loosely the probability of the existence of life beyond Earth. According to their analysis, just because life on Earth took shape early, endured and prospered doesn't mean the same process would naturally and inevitably occur elsewhere in the universe. Discovering life elsewhere, however, would be the only means of settling this debate. Unless the aliens find us first, of course.