University of Florida astronomers have discovered that a third of the planets around the most common stars in the galaxy could be in a goldilocks orbit close enough to hold onto liquid water – and possibly harbor life. The remaining two-thirds of the planets around these ubiquitous small stars are likely roasted by gravitational tides, sterilizing them.
Red dwarf stars, also called M dwarf or M-type star, the most numerous type of star in the universe and the smallest type of hydrogen-burning star. Red dwarf stars have masses from about 0.08 to 0.6 times that of the Sun.
Scientists now believe that 80 percent of the stars in our home galaxy, the Milky Way, are red dwarf stars. According to Jos de Bruijne, a scientist at the European Space Agency (ESA) who works on the galaxy-mapping Gaia mission, the current estimate is between 100 to 400 billion stars. This means 80 billion to 320 billion red dwarf stars and 24 billion to 96 billion with habitable zone planets.
The orbital eccentricity distribution of planets orbiting M dwarfs (PNAS)
Our yellow sun is a relative rare in the Milky Way. The most common stars are considerably smaller and cooler, sporting just half the mass of our sun at most. Billions of planets orbit these common dwarf stars in our galaxy.
Sagear and Ballard found that stars with multiple planets were the most likely to have the kind of circular orbits that allow them to retain liquid water. Stars with only one planet were the most likely to see tidal extremes that would sterilize the surface.
Since one-third of the planets in this small sample had gentle enough orbits to potentially host liquid water, that likely means that the Milky Way has hundreds of millions of promising targets to probe for signs of life outside our solar system.
Significance
The orbital eccentricities of exoplanets orbiting M dwarf stars may significantly affect their habitability but are unknown. We extract this eccentricity distribution using a sample of transiting planets orbiting M dwarfs detected by NASA’s Kepler Mission with stellar density measurements. We find planets in apparently single-transiting systems are typically more eccentric than planets in multiply transiting systems. The single- transit data prefer an eccentricity model composed of two distinct dynamically warmer and cooler subpopulations over three single-component models. With known planet demographics, we estimate the eccentricity distribution for the population of early- to mid-M dwarf planets in the local neighborhood, with implications for planetary formation and follow-up observations. Comparing our findings with similar studies for Sunlike stars suggests common dynamical excitation mechanisms.
Abstract
We investigate the underlying distribution of orbital eccentricities for planets around early-to-mid M dwarf host stars. We employ a sample of 163 planets around early- to mid-M dwarfs across 101 systems detected by NASA’s Kepler Mission. We constrain the orbital eccentricity for each planet by leveraging the Kepler lightcurve together with a stellar density prior, constructed using metallicity from spectroscopy, Ks magnitude from 2MASS, and stellar parallax from Gaia. Within a Bayesian hierarchical framework, we extract the underlying eccentricity distribution, assuming alternately Rayleigh, half-Gaussian, and Beta functions for both single- and multi-transit systems. We described the eccentricity distribution for apparently single-transiting planetary systems with a Rayleigh distribution, and for multitransit systems. The data suggest the possibility of distinct dynamically warmer and cooler subpopulations within the single-transit distribution: The single-transit data prefer a mixture model composed of two distinct Rayleigh distributions
over a single distrition. We contextualize our findings within a planet formation framework, by comparing them to analogous results in the literature for planets orbiting FGK stars. By combining our derived eccentricity distribution with other M dwarf demographic constraints, we estimate the underlying eccentricity distribution for the population of early- to mid-M dwarf planets in the local neighborhood.
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Why all the assumptions that life on these planets will be on the surface?
Typically because the surface has access to energy in the form of starlight, and in subsurface oceans the energy input is limited to geothermal vents.
I seem to remember someone did a simulation of what the temperature differential would be for a tidally-locked earth-like planet around a red dwarf. Because of winds between the two sides, it turned out to be only 40 degrees (don’t remember whether F or C).
There is also the problem these stars being flare stars, periodically roasting the planet with x-rays.
I wonder if that’s actually true. Suppose the cold side has even a single spot shielded from the wind, such as a meatier crater with high walls, or a tall extinct volcano with a deep pit at the top. If that single spot can reach cryogenic temperatures, then slowly more and more of the atmosphere will freeze out there, lowering the overall air pressure, reducing the winds further, and making the cold side colder over a larger area. I could imagine this feedback cycle eventually freezing out all of the atmosphere. That would then be a stable situation forever, because there is never again any wind to warm it.
On the other hand, a 3:2 resonance like Mercury, or a binary planet system tidally locked to each other, would probably prevent that outcome, and could keep the wind blowing forever. And that small 40 degree difference between the long day and the long night sounds like it could support life. If the star lasts a trillion years, that’s very interesting.
Though large x-ray bursts still sound like a problem, even for underground life. But life might survive at the bottom of a very deep ocean. I don’t know.
LOL. Autocorrect changed meteor to meatier. I wish we could edit our comments.
Now imagine some interstellar colonizers finding that and setting up mirrors to warm up that cold spot a bit to terraform the planet.
It’s a long way to cryogenic temperatures: -195 C -320 F for nitrogen, -183 C -297 F for oxygen. The crater / volcano would have to be tens of kilometers high.
The other interesting thing about this discovery is that Red Dwarf stars can be very old. The heaviest Red Dwarfs have life spans in the tens of billions of years while the lightest have life spans in the trillions of years. Both are longer than the known age of our universe but the implication is that life on any of those planets has a long runway.
The habitable zones are also much tighter to the star since their luminosity is much lower, which means a lot more periodic radiation since they’re incredibly variable.
Also, being so much closer to the star means a much greater chance of being tidally locked, so that one side is perpetually illuminated, and the other side dark.
However, you might have quite nice conditions along the terminator, if circulation could keep the atmosphere from freezing out on the dark side, without it being too windy. The continual cold wind would create a zone that was liveably moderate in temperature while having plenty of light.
In Hal Clement’s “Still World”, most intelligent life was ammonia based cryo-life living around red dwarfs, and humans were considered extremophiles on account of living at temperatures where water was a liquid, and surviving exposure to light that caused most species radiation burns.
I do wonder if we might find that most life lives under conditions we don’t survive under, using different chemistry. But chemical reactions occur so slowly at cryogenic temperatures that it’s tough to see how cryo life would have had time to evolve.
Not carbon based, but what about silicon ? I seem to remember something about silicon chemistry being less stable than carbon chemistry.
But aren’t these “habitable” red dwarf planets all tidally locked with one side scorched and dry and the other frozen and glaciated with only a narrow ring around the planet with liquid water?
Not necessarily all of them. Individual circumstances can prevent that, such as having a large satellite, or being resonance locked with a non 1-1 ratio. Mercury, for instance, is tidally locked to the Sun, but with a 3-2 ratio, it rotates 3 times for every 2 revolutions.
That can happen when the orbit is rather elliptical.
But, yes, I think it is the default condition, barring something preventing it.
Binary planets in red dwarf stars might be interesting, given they have a strong change to be tidally locked to each other, not to the star.
That means planets with long days and nights, but that aren’t with any side perpetually on day, night or twilight, resulting in probably less extreme and stormy weathers, and avoiding the atmosphere migration and freeze over on the dark side.