Life’s Constituents Around M-Dwarfs

by Paul Gilster on April 8, 2009

Kepler’s dust cover has now been jettisoned, meaning the search for extrasolar ‘Earths’ is not long from commencing. The cover stayed in place for so long because the spacecraft’s photometer had to make measurements of electronic noise that will later have to be removed from the science data. Mission engineers will now continue with the calibration process for several weeks using images of actual stars.

Our debates over the ‘rare Earth’ hypothesis will be getting firm data in short order because of Kepler. Three years from now, having had time to detect terrestrial-class planets in the habitable zone of their stars, confirm the detections and further examine the results, we should have at least a sense of how common such planets are. Finally we can move beyond informed speculation with the sort of hard data we need. And as far as the first terrestrial planet detection in the habitable zone, CoRoT may just beat Kepler to the punch.

Meanwhile, the astrobiological side of the ‘rare Earth’ debate gets more and more interesting with news that at least one important prebiotic chemical is in short supply around small M-dwarfs and their brown dwarf cousins. Have a look at the graph below, which clearly shows the gap in hydrogen cyanide (HCN) for small, cool stars as composed to Sun-like stars, whereas the baseline acetylene figures are roughly similar (and demonstrate that the method works).

cool_star_breakdown

Image: NASA’s Spitzer Space Telescope detected a prebiotic, or potentially life-forming, molecule called hydrogen cyanide (HCN) in the planet-forming disks around yellow stars like our sun, but not in the disks around cooler, reddish stars. The observations are plotted in this graph. Light wavelengths are shown on the X-axis, and the relative brightness of disk emission is shown on the Y-axis. The signature of a baseline molecule, called acetylene (C2H2), was seen for both types of stars, but hydrogen cyanide was seen only around stars like our sun. Credit: NASA/JPL-Caltech/JHU.

Ilaria Pascucci (Johns Hopkins) is lead author on the paper on this (slated for the Astrophysical Journal), describing her team’s investigation of seventeen cool and forty-four Sun-like stars with the Spitzer telescope’s infrared spectrograph. These are young stars presumably in the planet formation process, and none of the M-dwarfs and brown dwarfs in the mix showed a notable hydrogen cyanide signature.

Thus we add substance to the problematic nature of life around red dwarfs. Stellar flares have always been an issue, although some believe they could serve as a spur to evolution under the right circumstances. But a deficiency in hydrogen cyanide is more troublesome still, for HCN is a component of adenine, a basic element of DNA. Says Spitzer program scientist Douglas Hudgins:

“Although scientists have long been aware that the tumultuous nature of many cool stars might present a significant challenge for the development of life, this result begs an even more fundamental question: Do cool star systems even contain the necessary ingredients for the formation of life? If the answer is no then questions about life around cool stars become moot.”

We still haven’t found a planet in the habitable zone of an M-dwarf despite the Gliese 581 c finding — the planet is now believed to be far too hot for liquid water to exist on its surface. If life on Earth got its impetus from prebiotic molecules in the early protoplanetary disk, then a lack of same under these conditions makes M-dwarfs look less and less hospitable. That’s a downer for those of us fascinated with potential life around these dim stars, but of course the investigation of these matters is in its early stages.

{ 15 comments }

Hans Bausewein April 8, 2009 at 14:58

Aliens ate all HCN :)

o, no, then we would see their remains :(

Christopher L. Bennett April 8, 2009 at 16:21

Would alien life have to be based on DNA, though? I read somewhere that there are many possible molecules that could contain replicable genetic information and serve the equivalent function. We probably shouldn’t expect life on other planets to be DNA-based unless it was panspermically seeded from Earth or the same source as Earth.

Still, this could mean that it would be harder to terraform a red-dwarf planet for human habitation.

Adam April 8, 2009 at 17:08

Hi Chris

The other genetic molecules investigated so far as I know have all been based on amino acids, thus they too would be affected by a HCN shortfall. Of course the finding might mean that hydrocarbon based life might get a boost around red dwarfs, if such things are possible.

I don’t know if terraforming would be that much harder since most scenarios don’t assume large amounts of organic material anyway – who wants to clean an oil slick off a whole planet?

Blade O'grass April 8, 2009 at 19:44

If we share the galaxy with others then there will be no need for squabaling over recources. We can just chomp on the matter around the red dwarfs. Since 70\% of the galaxy is house for red dwarfs, it becomes a matter of how fast can your species can colonize the galaxy befor the other guy (er…creature). And if another species were to eventually surround some other species, wouldn’t the galaxy then resemble a giant amoeba, at least from a macroscopical point of view?

ljk April 8, 2009 at 20:46
Blade O'grass April 8, 2009 at 20:52

Oops! Yes, I realise that 100\% of the galaxy is house for for 100\% of the red dwarves.

Athena Andreadis April 8, 2009 at 22:18

Um… HCN is the potential precursor to amino acids, three of which go on to form inosine, which in its turn can become either adenine or guanine. But the emphasis is on potential — it’s not unique. Now if they had said that there is no nitrogen around red dwarves, that would be a far seriouser bottleneck!

Blade O'grass April 9, 2009 at 10:14

Thanks LJK. Those are awsome picts. What I was trying to suggest , is that with the lack of HCN (and the possibility of reduced life emerging from around the red dwarfs), that vast stores of unpopulated recources lay befor us (and others). If we can ever really colonize the galaxy, then for 70\% of the stellar objects out there we won’t be stepping on anyones toes…(free matter=free munchies). All we need do now is build the tin cans to get us there.

Didac April 9, 2009 at 10:43

HCN is not the nitrogen precursor of aminoacids (NH3 is) but it is the nitrogen precursor of purine and pyrimidine bases (Oro and Kimball, 1961). Purine and pyrimidine bases are needed in nucleotides (and so in DNA, RNA). By the way, HCN is needed in the abiogenesis process (the phase when biochemical building blocks are chemically synthesized). Once the life is established, there are ways far more efficient to synthesize nucleotides other than by direct assimilation of HCN.

spaceman April 10, 2009 at 11:34

I read in a recent Space.com article
(http://www.space.com/scienceastronomy/090409-sm-reddwarf-life.html)on life around M dwarfs something about the Kepler mission’s target list of stars has just been expanded to include more M dwarfs. Has anyone else heard anything about this? If it is true, it’s exciting.

People talk about the Mearth project and such as means of finding small planets around M dwarfs, but I have not heard much about the Kepler and Corot missions’ potential to find tiny planets around M dwarfs. Why is this? Is it because there is a general bias in favor of looking for Earth-sized planets at ~1 A.U. around G type stars?

What is the actual potential of Kepler and Corot for finding Earth-like planets around M type stars? Does anyone who frequents this site know?

Administrator April 10, 2009 at 13:41

I don’t know when the target star selection for Kepler was finalized, but it wouldn’t surprise me at all if at some point a decision had been made to keep an eye on M-dwarfs wherever possible. It’s true that we’ve had a bias for Sun-like stars for a long time, but recent studies have shown the potential for M-dwarfs in terms of possible habitability, and it would be useful indeed to get some sense for how common Earth-class worlds were around these stars in comparison with G and K stars. I think the potential for Kepler in finding such planets is high — the transit should be clearly defined if it’s there — but I’m not sure how CoRoT’s design scales to M-dwarf transits. Maybe someone else knows.

ljk May 26, 2009 at 13:03

While this is certainly fictional, perhaps we should consider the possibility
that just because some stars may not be friendly to organic Earth type life
does not mean that advanced species who are not limited to living on a
certain kind of planet may find red dwarf stars highly desirable:

http://eg.orionsarm.com/xcms.php?r=oaeg-view-article&egart_uid=47646874c1b82

Once again, we need to expand our search locations as well as our methods.
Old paradigms and attitudes may be causing us to miss all kinds of life forms.

ljk July 19, 2009 at 23:20

A Photometric Variability Survey of Field K and M Dwarf Stars with HATNet

Authors: J.D. Hartman (1), G.Á. Bakos (1), R.W. Noyes (1), B. Sipöcz (1,2), G. Kovács (3), T. Mazeh (4), A. Shporer (4), A. Pál (1,2), ((1) CfA, (2) ELTE, (3) Konkoly Observatory, (4) Wise Observatory)

(Submitted on 16 Jul 2009)

Abstract: Using light curves from the HATNet survey for transiting extrasolar planets we investigate the optical broad-band photometric variability of a sample of 27,560 field K and M dwarfs selected by color and proper-motion. A total of 3496 stars exhibit potential variability, including 95 stars with eclipses and 60 stars with flares.

Based on a visual inspection of these light curves and an automated blending classification, we select 1928 stars, including 79 eclipsing binaries, as secure variable star detections that are not high probability blends. We find that only 43 of these stars, including 7 of the eclipsing binaries, have previously been identified as variables or are blended with previously identified variables.

One of the newly identified eclipsing binaries is 1RXS J154727.5+450803, a known P = 3.55 day, late M-dwarf SB2 system, for which we derive preliminary estimates for the component masses and radii of M_1 = M_2 = 0.258 +- 0.008 M_Sun and R_1 = R_2 = 0.289 +- 0.007 R_Sun. The radii of the component stars are larger than theoretical expectations if the system is older than ~200 Myr. The majority of the variables are heavily spotted BY Dra-type stars for which we determine rotation periods.

This is the largest sample of photometric rotation periods for field K and M dwarfs published to date. Using this sample, we investigate the relations between period, color, age, and activity measures, including optical flaring, for K and M dwarfs.

We find that the fraction of stars that are variable with peak-to-peak amplitudes greater than 0.01 mag increases exponentially with the V-K_S color such that approximately half of field dwarfs in the solar neighborhood with M ~< 0.2 M_Sun are variable at this level. Our data hints at a change in the rotation-activity-age connection for stars with M <~ 0.25 M_Sun (Abridged).

Comments: 33 pages, 26 figures, 17 tables, submitted to AJ

Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

Cite as: arXiv:0907.2924v1 [astro-ph.SR]

Submission history

From: Joel Hartman [view email]

[v1] Thu, 16 Jul 2009 20:07:27 GMT (1183kb)

http://arxiv.org/abs/0907.2924

ljk September 10, 2009 at 12:50

Radiatively heated, protoplanetary discs with dead zones. I. Dust settling and thermal structure of discs around M stars

Authors: Yasuhiro Hasegawa, Ralph E. Pudritz

(Submitted on 9 Sep 2009)

Abstract: The irradiation of protoplanetary discs by central stars is the main heating mechanism for discs, resulting in their flared geometric structure. In a series of papers, we investigate the deep links between 2D self-consistent disc structure and planetary migration in irradiated discs, focusing particularly on those around M stars.

In this first paper, we analyse the thermal structure of discs that are irradiated by an M star by solving the radiative transfer equation by means of a Monte Carlo code.

Our simulations of irradiated hydrostatic discs are realistic and self-consistent in that they include dust settling with multiple grain sizes (N=15), the gravitational force of an embedded planet on the disc, and the presence of a dead zone (a region with very low levels of turbulence) within it. We show that dust settling drives the temperature of the mid-plane from an $r^{-3/5}$ distribution (well mixed dust models) toward an $r^{-3/4}$.

The dead zone, meanwhile, leaves a dusty wall at its outer edge because dust settling in this region is enhanced compared to the active turbulent disc at larger disc radii.

The disc heating produced by this irradiated wall provides a positive gradient region of the temperature in the dead zone in front of the wall. This is crucially important for slowing planetary migration because Lindblad torques are inversely proportional to the disc temperature.

Furthermore, we show that low turbulence of the dead zone is self-consistently induced by dust settling, resulting in the Kelvin-Helmholtz instability (KHI). We show that the strength of turbulence arising from the KHI in the dead zone is $\alpha=10^{-5}$.

Comments: 19 pages, 20 figures, 3 tables, accepted for publication in MNRAS

Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

Cite as: arXiv:0909.1734v1 [astro-ph.EP]

Submission history

From: Yasuhiro Hasegawa [view email]

[v1] Wed, 9 Sep 2009 16:08:47 GMT (1241kb)

http://arxiv.org/abs/0909.1734

Adam November 6, 2009 at 10:39

Wait a minute. Did I hear them say that there just wasn’t a “notable” HCN signature? Did they actually find it or not? Even if it’s not in general abundance, wouldn’t the inward migration of volatiles from the outer system during planetary formation serve to concentrate even meager quantities of HCN? Can someone give me the heads up on that?

Comments on this entry are closed.

{ 1 trackback }