Only recently has the idea of habitable planets around red dwarf stars taken hold. But it’s a fascinating one, especially if you take a look at the potential window for life to develop on such worlds. M-class red dwarfs live anywhere from 50 billion up to several trillion years, a vast stretch compared with our own Sun’s projected ten billion years. And with 75 percent of main sequence stars thought to be red dwarfs, the hunt for life can be expanded enormously if we add red dwarfs to the mix.
But getting a stable environment for that life to develop is another matter, for planets in the habitable zone around such stars would be close enough to their primaries to be tidally locked, with one side always in sunlight, the other in darkness. The thought of a frozen dark side and a scalded day side isn’t pretty. It wasn’t until the late 1990s that models of heat transport within the atmosphere developed that could even out these stark extremes. Now it looks as though habitable worlds could exist around M dwarfs, but just where do we look?
A new study tackles this question from the standpoint of ultraviolet light and its potential effects on stellar habitable zones. Last April we looked at a paper on this topic by the same team, led by Andrea Buccino (Instituto de Astronomía y Física del Espacio, Buenos Aires). Now Buccino and colleagues are looking at the UV emitted by solar flares on M dwarfs, using their model of an ultraviolet habitable zone to study three dwarfs in particular: GL 581, Gj 849 and Gl 876. Their data come from observations made by the International Ultraviolet Explorer (IUE) satellite.
Ultraviolet is a critical parameter because enough of it can inhibit photosynthesis and destroy DNA. In the right amount, however, it can be a significant energy source for synthesizing biochemical compounds. In the case of the three planets studied, however, the liquid water habitable zone and the ultraviolet habitable zone do not overlap, with the UV much weaker than needed to trigger the formation of complex molecules at distances where liquid water could exist on a planetary surface.
But get this: Solar flares could provide the needed trigger, and many M dwarfs are flare stars. From the paper:
In contrast to what it has been believed for a long time, the ?are activity in M stars could play an important role in the origin of life. Moderate ?ares could trigger the biogenesis processes while the effect of strong ?ares, that are less frequent, could be mitigated by the production of abiotic ozone and the fact that only one face of the hypothetical terrestrial planet within the [liquid water habitable zone] will receive the damaging UV radiation.
M dwarfs with moderate flare activity thus become our most likely candidates for habitable worlds around such stars. Indeed, red dwarfs with no flare activity would need alternate sources of energy to start biogenesis. That conclusion marks the continuing reshaping of the red dwarf model, since it used to be thought that such flares would effectively prevent life from ever forming. The paper is Buccino et al., “UV habitable zones around M stars,” submitted to Astronomy & Astrophysics and now available as a preprint online.
So, hypothetically speaking… How many stars are we now considering “somewhat, potentially, maybe one in a million chance”-habitable?
Actually, what is the current estimates of stars in the Milky Way? When I was younger it was 100 billion, more recently I think I have seen 200-400 billion… and within what distance (the 80-100,000 light year distance).
Reading about findings like this always gets me thinking about how likely it is to run into a neighbor. Imagine if you were in the middle of Montana…. actually, I’m going to do some number crunching. I’ll post back with my ignorant findings later. :)
-Zen Blade
One problem with tidally locked planets——doesn’t that pretty much preclude a magnetic field? No magnetic field, no protection from flares….
At least on the red dwarf side.
My understanding is that even the slow rotation — once an orbit — of a tidally locked planet would be enough to create the needed magnetic field, given a molten interior. But I’ve also written Dr. Buccino for a comment.
As I understand it, the magnetic dynamo is more dependent on the temperature gradient in the planet’s interior than the planet’s rotation.
The liquid water habitable zones in that paper seem rather close to the star – back-of-the-envelope calculations I’ve done suggest that the temperature of a spherical planet at that kind of distance would be several hundred degrees Centigrade. If the LWHZ is further out, larger flares would be required!
Zen, I always go with the 100 billion figure for stars in the Milky Way, obviously the more conservative choice, but it does look like that number is routinely doubled depending on the source, and sometimes does get cited at 400 billion. I’d appreciate anyone’s pointer to a recent paper analyzing this in light of the latest work on the population of brown dwarfs.
I got distracted today and ended up spending some time trying to calculate the number of “points with life” in the galaxy using simple distance and frequency numbers.
example: 10 light years = radius of a local circle or volume from Earth [RL]. If we assume there is only one habitable point within this area or volume, and if we assume this area or volume is representative of the overall galaxy, how many points of life would there be? Or, to flip this around… if we find life within X light years of earth at a certain frequency, what does this frequency tell us about life throughout the galaxy.
Sorry in advance for the lack of easy to follow symbols and subscripts (can’t figure how to include them).
Area = pi*r^2
Volume =4/3*pi*r^3 (when r 1000 LY).
Estimates:
Area/Volume of Galaxy when R=40,000 LY (R total, Rt)
Total Area = 5.03×10^9 LY^2
Total Volume = 2.68×10^14 LY^3
For a given “Rl”,
total # of points in 2 dimensions = Total Area/local Area = 5.03×10^9 / Rl^2
total # of points in 3 dimensions = Total Vol./local Vol. = 5.03×10^12 / Rl^3
Therefore, when R Local [Rl] = 16 LY,
total points in a plane of the galaxy (2D) = 6.25×10^6.
Total points in the galaxy (3D) = 2.93×10^8.
When Rl = 128 LY, total pts (2D) = 97,639. Total pts (3D) = 573,708
When Rl = 750 LY, total pts (2D) = 2844. Total pts (3D) = 2844
When Rl = 999 LY, total pts (2D) = 1604. Total pts (3D) = 1203
When Rl = 1001 LY, total pts (2D) = 1600. Total pts (3D) = 1600**
When Rl = 3,648 LY, total pts (2D) = 120. Total pts (3D) = 120
**change in shape of local volume; change in formula.
By now, I may just be on a personal exercise of futility, but what struck me is the distance and how the places of life and distance actual measure up. If life is common, well maybe there are just too many locations and an intelligent species may not even bother exploring more than a few hundred light years in any direction. But, on the flip side if there aren’t very many sites of life, why bother exploring much, if at all….
Too much life, or not enough life could both lead to the same outcome… relative isolation of intelligent species.
This reminds me of a chemical problem of how life might first start… if use look at the “RNA World”, as you begin to string together an RNA oligonucleotide–one of your first information-containing components, it isn’t too long before possible combinations become so overwhelming as to make the initation of life highly improbable. If oligonucleotide is length 100, then there are 4^100 possible combinations.—How long would it take to find something productive?
-Zen Blade
Hold on, there seem to be considerations you’ve missed. First, Venus is very much hotter than earth would be in the same orbit: a massive greenhouse effect has taken hold. So why can’t there be planets too far out (and potentially too cold) from red dwarf primaries which have temperatures about right? Also suppose a planet is in this tidally locked zone, but is itself in orbit around a Jupiter-type planet. Would life be possible then? The galaxy is very very big, and a lot of things seem possible. I don’t say these combinations are common.
Zen, sorry to take a contrary position, but I think your calculation went astray when you started, “if we assume…”. Your calculation in effect asserts a probability of life give ‘x’ where x is some volume of space, number of planets, or something similar. Unfortunately that probability is not known and any extrapolation, while very interesting speculation, doesn’t deliver results.
Probability theory as I understand it (university for me was a few decades ago), when based on empirical data, the evidence, tells us that the expected value of planets with life is… 1. I know, that’s very obvious and boring, but it does sum up the data. We can actually know that it is 1 without knowing the probability of life arising or even the sample size. That is, while we don’t know the probability, of all the planets/exoplanets we’ve looked at we’ve found life on one of them — that would be us of course.
Arguably we have not done a very thorough investigation so far, and there promises to be great strides in the coming years, but 1 sums up the situation today. We’re still not even convinced there isn’t/wasn’t life on Mars.
Ron,
My point is not to come up with some, “there are this many habitable spots”… rather, my purpose was to get a feeling for what “x number of habitable spots” would actually mean in terms of actual distances.
For example, if there are millions of habitable spots (from on now I will say planets for simplicities sake), then we will likely finds numerous planets within a couple hundred light years that are habitable. However, if the number is on the order of a few hundred planets or a few thousand planets with life… then the distances are likely to be quite great.
-again, I made a lot of assumptions, the most important one (from my perspective) being the idea of a homogenous galaxy with an evenly distributed set of habitable planets. The actual number doesn’t matter to me, just the idea of what the physical manifestation of that number… and how a civilization might respond to that physical manifestation.
Actually, I don’t read enough, but this is a subject I would be interested in reading about if someone has done some real work on this and published something.
-Zen Blade
As for long oligonucleotides and improbabilities of useful sequences, the odds are probably a lot better than they seem. The amino acid sequences of many, many proteins differ by as much as 80% (or more in some cases) and yet they still perform the same functions. Once a useful sequence is found mutation and selection will find more useful sequences in a shorter time than purely random saltation.
We know enough about proteins now to say that there’s a lot of valid and useful sequences in “protein space”.
Adam
Andrea Buccino, lead author on the paper under discussion here, was kind enough to respond to my query re magnetic fields and red dwarf habitability:
”You have to remember that our habitability criteria are based on the Principle of Mediocrity. It means that planets where life could emerge would be similar to our own Earth.
“The magnetic field in the Earth is generated by the dynamo mechanism,
which is related to the convection and differential rotation in the liquid
core. Steenbeck et al. (1966) developed the mean field theory, where
they obtained an accepted solution for the magnetohidrodynamic (MHD)
equations. They found that the dynamo proccess depended on the turbulence in the convection layer. Therefore, the generation of magnetic field in associated to microscale phenomena. I think that a hypothetical terrestrial planet tidally locked around an M star could mantain a dipolar magnetic field if it had a liquid core where turbulence and differential rotation were present. The macroscale velocity field is not ‘responsible’ for the magnetic field.”
Many thanks to Dr. Buccino for the additional information.
Odd Little Star has Magnetic Personality
http://www.spaceref.com/news/viewpr.nl.html?pid=24186
“A dwarf star with a surprisingly magnetic personality and
a huge hot spot covering half its surface area is showing
astronomers that life as a cool dwarf is not necessarily
as simple and quiet as they once assumed.”
THE MOUSE THAT ROARED: PIPSQUEAK STAR UNLEASHES MONSTER FLARE
GREENBELT, Md. – On April 25, NASA’s Swift satellite picked up the
brightest flare ever seen from a normal star other than our Sun. The
flare, an explosive release of energy from a star, packed the power of
thousands of solar flares. It would have been visible to the naked eye
if the star had been easily observable in the night sky at the time.
The star, known as EV Lacertae, isn’t much to write home about. It’s a
run-of-the-mill red dwarf, by far the most common type of star in the
universe. It shines with only one percent of the Sun’s light, and
contains only a third of the Sun’s mass. At a distance of only 16
light-years, EV Lacertae is one of our closest stellar neighbors. But
with its feeble light output, its faint magnitude-10 glow is far below
naked-eye visibility.
“Here’s a small, cool star that shot off a monster flare. This star has
a record of producing flares, but this one takes the cake,” says Rachel
Osten, a Hubble Fellow at the University of Maryland, College Park and
NASA’s Goddard Space Flight Center in Greenbelt, Md. “Flares like this
would deplete the atmospheres of life-bearing planets, sterilizing their
surfaces.”
For more information and images, please visit on the Web:
http://www.nasa.gov/centers/goddard/news/topstory/2008/pipsqueak_star.html