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Lake-front property, expansive view of faint, red sun that never sets

From right to left: Courtney Dressing, Dave Charbonneau and yours truly at the live CfA press conference. Photo by Kris Snibbe/Harvard Staff Photographer
Yesterday, I had the honor of participating in a live press conference today at the Harvard Center for Astrophysics (CfA). The event was to announce new findings by third-year graduate student Courtney Dressing and her advisor Dave Charbonneau, who studied the occurrence of planets around M dwarfs in the Kepler field. Dear Sara Seager, check it out! A woman with not only a big exoplanet press announcement, but a HUGE exoplanet press announcement! (But, yes, we need more).

Sound familiar? If so, it's because Jon Swift and I made a related announcement last month at the AAS meeting. But while we focused ont he bulk occurrence rate, finding 1.0 +/- 0.1 planets per M dwarf, Courtney focused on Earth-like planets. By Earth-like she means, "planets the size of the Earth that receive a similar amount of sun light as our planet." (As an aside, Jon and I were very much relieved and excited that Courtney's statistical analysis matched our result on the bulk occurrence rate.)

Her big results are

  • 6% of M dwarfs (=red dwarfs) have Earth-like planets.
  • This means that there are at least 6 billion Earth-like planets in the Galaxy since M dwarfs comprise 7 out of 10 of the Milky Way's stars
  • The nearest Earth-like planet is around an M dwarf within 13 light years of the Earth. Which one? We don't know...yet. We need to start searching, like, yesterday IMO.
  • At 95% confidence, there is a transiting Earth-like planet around an M dwarf within 100 light years.

Here's the CfA press release.

Here's a preprint of Courtney's paper, which will very soon be accepted by ApJ (referee report was positive and has been responded to).

Slide from Courney Dressing's press announcement showing the amount of "sun light" received by the planets around Kepler's red dwarfs. The locations of Mars, Earth, Venus and Mercury are shown along the top. Three of the planets around Kepler's red dwarfs are squarely in the "goldilocks zone".
Have done several of these types of press conferences over the past couple of years, I've started recognizing a pattern in the Q&A with the press. It goes a little something like this:

Astronomers: 6% of M dwarfs have Earth-sized planets in the HZ! (out of breath from all the hard work)
Reporters: But come on, can life really emerge on planets around M dwarfs?! What about flares and tidal locking and bears, oh my? (Ed. note: Okay, I added that third problem)
Astronomers: Ummmm...did we mention all the Earth-sized planets we found with temperate equilibrium temperatures? 

First, I'll admit that it's the fault of astronomers for playing it fast and loose with the term "habitable" in reference to the locations of certain planets around other stars. The habitable zone is an extremely idealized concept referring to the region around stars where the incident sun light results in planetary temperatures that would be like the Earth's. But this is under the assumption that the planet has an Earth-like orbit (low eccentricity), an Earth-like atmosphere (albedo), and a nice solid surface where liquid water could pool into lakes and oceans and the like. So reporters are correct to be skeptical.

Thus, when an astronomer says "habitable zone," there's no reason to conclude that the planet is inhabited, or that it even could be inhabited (despite what some astronomers believe). Instead, when you hear the term you should think "possible location around the star where, if a myriad set of conditions are just right, a planet could have liquid water on the surface." Habitable zone is just much easier to say. Also, the habitable zone is something that is easy to calculate based on the parameters of planets discovered by various techniques. We bag 'em, the astrobiologists tag ' help us understand whether they could truly be habitable.

So my first point is for the astronomers. We need to be more nuanced when tossing around notions of habitability. My second point is to the reporters. The question "Are these planets truly habitable" is pretty much impossible to answer right now. Why? Because we don't even know the conditions for habitability on our own planet! Here's a long, yet incomplete list of factors/questions that may or may not be important for the emergence of life on Earth:
  • Our Moon maintains the Earth's moderate obliquity (axial tilt). Mars undergoes large obliquity swings because it has no moon, which wreaks havoc with its weather
  • We have plate tectonics to maintain a carbon-silicate cycle, which keeps CO2 in a stable equilibrium. Maybe. We think. 
  • If plate tectonics are necessary, is the high water content of Earth's mantle necessary for plate tectonics?
  • If the Earth formed "dry" then how was water delivered? 
  • Do we need a large ocean to maintain thermal inertia?
  • Is it important that we have just the right amount of water so as to not cover all landforms?
  • Is dry land necessary?
  • Is Jupiter a friend or foe? Does it hoover up comets or toss asteroids in?
  • Why do we have a hydrogen-poor atmosphere?
  • Is water the only suitable solvent for life?
  • Is it important that we lack a close stellar binary companion despite ~50% binarity of stars Galaxy-wide?
  • Do we need an especially "calm" sun?
  • Do we need a low eccentricity?
  • Earth is not too large as to have ended up as a mini-Neptune
  • Earth is not too small to end up like Mars with high atmospheric escape
  • What about Milankovitch cycles?
  • Do we need our nickel-iron core for magnetic field generation? 
This is just a partial list that I was able to come up with while Google chatting with Prof. Jason Wright. What did we forget?


Andrew Howard said…
Great post and congratulations especially to Courtney!

A word of caution about over-restricting the habitable zone with "rare-Earth" reasoning (requiring too many specific characteristics of the Earth). On this point I particularly like excerpt below from Chyba & Hand, 2005, Annual Reviews of Astronomy & Astrophysics, 43, 31

"A second example of “rare-Earth” reasoning concerns conclusions drawn from the important discovery of the obliquity-stabilizing effect of Earth’s Moon (Laskar & Robutel 1993; Laskar, Joutel & Robutel 1993). The inference is made (Ward & Brownlee 2000, Gonzalez & Richards 2004) that complex life must therefore be rare, on the grounds of the assertion that Earths with Moon-size satellites must be rare, and that in the Moon’s absence wild obliquity fluctuations would occur that would render the environment too inconstant for the evolution of complex or intelligent life. [There is now observational evidence that large planetesimal collisions in other solar systems are common at the end of planetary accretion (Rieke et al. 2005), but, of course, there are currently no statistical data about the frequency or nature of planet–moon combinations that may result.]

But again one must ask what Earth may have been like had the Moon never formed—not what the Earth would look like if today one somehow plucked away the Moon. Laskar & Robutel (1993) show that Moonless Earths rotating with periods <12 hr may be stable against chaotic obliquity fluctuations for a large range of obliquity angles. Of course the current Earth’s period is 24 hr, so if we pluck away the Moon today chaos sets in. But if the Moon had never formed, what would Earth’s rotational velocity have been? A simple angular momentum conservation calculation shows that if one tidally evolves the lunar orbit back in time from its current position at 60 R⊕ to an early orbit at 10 R⊕, Earth’s day would have been about 7 hr long, giving an Earth likely stable against chaotic obliquity fluctuations. Touma’s (2000) simulations of the Earth–Moon system take Earth’s initial rotation period to be 5.0 hr, with the Moon at 3.5 R⊕. Of course, this does not demonstrate that Earth’s rotational period would have been this short had the Moon never formed; it is difficult to estimate Earth’s primordial rotation in the absence of the putative Moon-forming impact [see Lissauer, Dones & Ohtsuki (2000) for a discussion of the issues]. But it shows the arbitrary nature of reaching conclusions about Earth’s rarity by plucking away the Moon today, rather than, say, shortly after lunar formation."

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Sarah Rugheimer said…
It is important to distinguish between what is habitable for complex versus microbial life. For Earth-like life at least, those two conditions are very different and it’s difficult to say how evolution would adapt to different conditions. Most of the factors in this list wouldn't be relevant for microbial life even on Earth if those things were changed today.

The moon - may not be a deal breaker as Andrew points out. Jupiter as you mentioned is probably neutral since it both protects us and throws stuff in. Size of the planet matters in that we assume currently you need a solid surface. Plate tectonics – probably useful to have a cycle for long term climate stability, but life could arise for the some time without it since we have evidence for life very quickly after Earth cooled. Norm Sleep has many papers on this and here is a great conversation he has about these things, including land fraction coverage and habitability in general ( & Ray Pierrehumbert estimates as long as there is 10% surface fraction of water you will have similar climate and climate cycling as Earth. A recent paper by Abbot et al. (2012) also claims that the surface fraction of water doesn't have a large effect on habitability. Activity of star - if the life is under water or ground this doesn't matter at all, and it's unclear whether it would be harmful since there are examples even of animal life on Earth which have high radiation tolerances. Is water the only solvent - Steve Benner would say no (Benner et al., 2004), though water is very abundant compared to some of the other proposed solvents! Low eccentricity - depends on how much time it spends in the HZ (Dressing et al. 2010) and probably extremophiles would do better than complex life. Magnetic field - probably helpful, but less important for life sheltered under water or a layer of soil. Hydrogen in the atmosphere I've not really heard of as being relevant for life other than extending the habitable zone outwards. Binaries - I think the main problem is stability of orbits but if the binary is wide enough this isn't an issue (Eggl 2012, Kaltenegger & Haghighipour 2013).

In the end I think you hit on a very important point. Just because a planet is habitable doesn't mean that the planet is "100% likely to have life. Like you said, we just don't know until we have more information about the planetary context. The only way we'll begin to answer these questions is by detecting biomarkers in the atmospheres of a variety (or lack thereof) of planets and exploring other habitable environments up close in our own solar system like on Titan, Mars, Europa and Enceladus. It's also useful I think to note that this notion of a habitable zone around other stars only is relevant for remote detectability of features in the atmosphere. Europa in our own solar system is a prime example of a habitable environment that we would never detect in another star system since there is no interaction between the life and that atmosphere. Furthermore life built on a different biochemistry would have different signatures that currently are hard to predict and unambiguously distinguish as coming from life. So the HZ concept doesn't mean that's the only place life could be, just that since we know life on Earth uses liquid water, it's the best first place to start. Even Earth-type life could thrive in protected environments far outside the traditional HZ such as in Europa.

One thing that you didn't mention on your list but could be important and is observable is the C/O ratio. If there is more carbon than oxygen the O would be taken up by CO and CO2 and then there would be none left to form silicates. SiC would take the role of silicates and they are very durable and unlikely to weather, making a climate cycle unlikely (Kuchner & Seager 2005).

Those are just some of my thoughts! Great post! :)

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