|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).
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 'em...er...to 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?