Will the first ‘super-Earth’ in the habitable zone of its star be found around a red dwarf? An M5-dwarf with both mass and radius about a quarter that of the Sun would have 1/200th Sol’s luminosity. That’s interesting for transit purposes, for a planet in the habitable zone around this star would be close in indeed, some 0.074 AU out, with an orbital period of 14.8 days. Its transit probability would correspondingly be raised by a factor of three compared to the Earth-Sun system.
The result, as laid out by the transit survey called MEarth: Detecting such planets should be possible from the ground. Take a look at the live video of what MEarth is doing. Based at the Fred Lawrence Whipple Observatory on Mt. Hopkins in Arizona, the team works with 1976 nearby red dwarfs, visiting each repeatedly in hopes of snaring an ongoing transit, whose information would then be routed to larger instruments for confirmation. They’re looking at targets spread over the entire celestial northern hemisphere and varying the parameters of each observation to the individual target star. And for this survey, the smaller stars are best:
…the most favourable targets for such a transit survey are, in fact, the smallest stars: although these are intrinsically fainter, the reduced count rates are compensated by having deeper transits, and their faintness increases the number of suitable comparison stars available for a given field-of-view. It is important to recall that for small field-of-view observations of single targets, the noise in the comparison light curve can become an important, or even dominant, contributor to the total noise budget. We therefore further choose to concentrate on the smallest stars…
Small, low luminosity stars with possible planets in a habitable zone close enough to the parent to permit ground-based detection — these are exciting thoughts as we tune up our transit methods and await the launch of Kepler. The small radius of M-dwarfs means that any transiting super-Earth is going to block that much more starlight, throwing a clearer transit signature. We can add in the fact that the small stellar mass coupled with a close-in planet also offers a much clearer radial velocity signature for follow-ups.
Working at infrared wavelengths just longer than visible light, MEarth’s eight robotic telescopes will need a total of two years to complete the survey of its target stars. And there’s this on potential findings:
The design study indicates that a yield of 2.6 habitable zone super-Earths would be predicted if the true occurrence of these planets was 10% around our targets, with larger and closer-in planets being easier to detect. A null result would limit the occurrence of > 2 R? super-Earth planets in the habitable zones of late-M dwarfs to be < 17% at the 99% confidence level, a result that again becomes a stronger limit for closer-in planets.
Remember the key advantages of transits. Measuring the planet’s size by examining the amount of light it subtracts from the star’s light can, when combined with radial velocity data, determine the planet’s true mass and help us work out its density. The James Webb Space Telescope, scheduled for a 2013 launch, might then be able to give us spectral information as starlight filters through a planetary atmosphere. Science News has a good story on MEarth, from which this quote by David Charbonneau (Harvard-Smithsonian Center for Astrophysics), whose team is behind the MEarth project:
“My goal is very much to learn about the robustness of life in different stellar environments. If we find planets in the habitable zones of low-mass stars, and determine that these planets have all the right building blocks for life—for example that they are rocky, are at room temperature and have liquid water—but find no life upon them, that would be a very interesting result indeed.”
There are all kinds of reasons why M-dwarfs might be hostile to life, including the consequences of flare activity on closely orbiting planets (not to mention the nature of super-Earths themselves). But one step at a time, as we first try to determine just how common such worlds are. The paper is Irwin et al., “The MEarth project: searching for transiting habitable super-Earths around nearby M-dwarfs,” appearing in Proceedings of the 253rd IAU Symposium: “Transiting Planets” (May 2008, Cambridge, MA) and available online.
Out of curiosity, what is the effect of rings, many multiple moons, or binary planets when inferring the planet size? Is it possible they could throw the result off significantly? Are they distinguishable?
At the close distances of the red dwarf habitable zone, moons would likely be unstable because of tidal forces. As for the effects of large moons on transits, a moon would cause the time of transits to vary, and cause variations in the duration of the transits.
Hi Folks;
The really cool thing about interstellar and intergalactic fusion fuel gases is that only about 10 percent to 20 percent of it at most, has been incorporated into stars. It may even be the case that much, although probably only a fraction of, the so called cold dark matter is hydrogen and helium. This is potentially great news regarding any future era of star formation with renewed vigor.
Some red dwarf stars, those of the M class with the lowest mass, may according to some models shine with more or less constant output or steady output for 100 trillion years. The good news is that most stars are M-class dwarfs. Some other models suggest that a shorter life of about 10 EXP 13 to 10 EXP 13.5 may be the upper limit.
The possibility that there just might be red dwarfs that exist today, that will still be shinning 10 trillion years from now, and which from this future date, might shine in theory for another 90 trillion years, fills my mind with amazement. Even at 0.1 C, imagine how far we could travel in 10 trillion years: a mere one trillion light years not taken into account the expansion of space time. Taken such expansion into account, the distance traveled from the Milky Way Galaxy will be several orders of magnitude greater yet. At near light speed travel, the distances traveled become commensurately greater. With FTL travel, they become potentially enormously greater.
With all of this in mind, I am becomming ever more convinced that we will become a star faring civilization and one of cosmic proportions. I think habitable zones around red dwarf stars will likely play a key role here. Thus I think it is never to early to get to the meat of studying habitable zones around red dwarf stars in general and in detail.
Thanks;
Jim
Hi James
Red-dwarf evolution, in the long term, has been examined in detail by Greg Laughlin, Fred Adams and Peter Bodenheimer. As Greg discussed on his systemic blog a while back as the Galaxy ages lower and lower mass stars brighten and move off the Main Sequence, keeping the overall light steady for 800 billion years – then it goes into gradual decline over the next ~9 trillion years before sputtering out. But that’s assuming fusion as the sole power-source. Red-dwarfs are very efficient fusion reactors since they fuse 98% of their hydrogen before fading out as helium dwarfs. The Sun, for comparison, only fuses ~8% before it goes off the Main Sequence, then rapidly burns off and blows off the remainder – about 54% ends up as a carbon/oxygen white dwarf cinder, with the rest blown away.
If – a big if – proton-decay can be catalysed via some ultra-stable super-symmetric particle or mini-black holes, then the picture changes. Consider the 0.1 solar mass red dwarf – if it radiates at its usual 1/1200th of a solar luminosity then its total mass-energy could power it for almost 1,800 trillion years. Using all the Galaxy’s mass-energy (3 trillion solar masses) for power then its 30 billion solar luminosity would last ~1,500 trillion years. However most of the Galaxy’s glow is from overly wasteful O & B stars, which might look pretty, but are otherwise rather wasteful. Especially when they lock up mass as black-holes. A more conservative Galactic energy budget might last 10-100 times longer by discouraging O & B star formation.
Now this long-term view might seem tangential to detecting red-dwarf planets, but logically red-dwarfs are the places to colonise for long-term civilizations – at least until They get around to turning stars into proton-decay reactors. The more we learn about red-dwarf planets, the more we’ll know about the potential real estate of the Cosmic Elders.
James,
yes, I understand that only a small fraction of the hydrogen of the universe has been used up (I am not sure about the exact %, 7 orso?, hardly a dent in the overall supply) and that the stelliferous stage of the universe can last some 100 trillion years (or even more).
I would rather think that a typical M dwarf (20 % solar mass, 1% total luminosity) will last some 1 trillion years and the smallest (8% solar mass) up to 6 trillion years, but you may know more than I.
The other side of this coin is that we will run out of solar type stars quite rapidly (in cosmic terms): when our universe is some 100 billion years old, still extremely youthful, red dwarfs will probably dominate the scene almost completely and solar type stars will have become quite rare.
“we will become a star faring civilization and one of cosmic proportions. I think habitable zones around red dwarf stars will likely play a key role here. Thus I think it is never to early to get to the meat of studying habitable zones around red dwarf stars in general and in detail.”
Or maybe there is an interesting alternative for a (very far) future (very) advanced civilization (level K3 or 4): maybe we could consider all those very long-lived red dwarfs as a ‘cosmic reserve’ or ‘spare parts ‘; merge 4 average red dwarfs together and you have a solar mass star. Wasteful, of course, but pleasant.
Review: The Crowded Universe
The field of extrasolar planets has exploded in the last 15 years as astronomers have discovered hundreds of such worlds around other stars.
Jeff Foust reviews a book by a leading scientist on the topic that reviews the science, missions, and policy developments during this time.
http://www.thespacereview.com/article/1305/1
New Worlds: Evaluating terrestrial planets as astrophysical objects
Authors: Caleb A. Scharf (Columbia), David S. Spiegel (Princeton), Mark Chandler (Columbia), Linda Sohl (Columbia), Anthony Del Genio (NASA/GISS), Michael Way (NASA Ames/GISS), Nancy Kiang (NASA Ames/GISS)
(Submitted on 16 Feb 2009)
Abstract: Terrestrial exoplanets are on the verge of joining the ranks of astronomically accessible objects. Interpreting their observable characteristics, and informing decisions on instrument design and use, will hinge on the ability to model these planets successfully across a vast range of configurations and climate forcings.
A hierarchical approach that addresses fundamental behaviors as well as more complex, specific, situations is crucial to this endeavor and is presented here.
Incorporating Earth-centric knowledge, and continued cross-disciplinary work will be critical, but ultimately the astrophysical study of terrestrial exoplanets must be encouraged to develop as its own field.
Comments: Submitted as a White Paper to the 2010 Astronomy & Astrophysics Decadal Survey
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:0902.2755v1 [astro-ph.EP]
Submission history
From: Caleb A. Scharf [view email]
[v1] Mon, 16 Feb 2009 18:33:15 GMT (394kb)
http://arxiv.org/abs/0902.2755