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Shrunken Planets

Turns out, Earth-sized planets in habitable zones around low-mass stars are not as rare as Kepler initially led astronomers to believe (not Kepler the guy--he's dead--but Kepler the exoplanet-scouting satellite. For a description of how Kepler finds planets, stay after class). Initial analysis of the data found only one Earth-sized planet habitably orbiting an M-dwarf star, but by changing the way the data were interpreted, a group of astronomers led by Philip Muirhead at Caltech has determined that there are six Earthy planets in nonboiling orbits.

What's an M-dwarf star?
It's a low-mass, relatively cool star that lies on the main sequence. They have between 8% and 60% of the mass of the sun and are between 60% and 10% as large. They are the most common stars around, and, turns out, they also like to grow planets.

This is convenient, right?

The entire and extended series! On DVD!
A three-disk set! Source.
Well, not exactly. M-dwarfs' ubiquity is useful for finding planets, but it's not so useful for finding out things about planets. See, we know a lot more about stars that are like our Sun than we do about stars that are, like, smaller. We--and I don't know if you know this and I don't want to alarm you--live next to the Sun. The proximity makes it much easier to study. And since we can apply what we learn about the Sun to stars that are like the Sun, we know a lot about these other stars. We do not live next to tiny, cool stars, so their properties are harder to pin down.

But you started out talking about planets. Why are you talking about stars?
The information astronomers dispense about planets is not deduced directly from observations of the planets, but from observations of the stars around which they orbit. In order to know about the planet, they have to know about the star and deduce knowledge about the planet.

To put it briefly, Kepler finds planets by watching for stars whose brightness dips with regularity, as that dimming could be evidence of planets passing in front of them and blocking part of the light.

The light itself--which, keep in mind, is all astronomers ever get to draw all the conclusions they've drawn about the universe--comes from the star.

What did this group most want to find out about the planets?

  • radius
  • mass
What did they need from the stars?
  • effective temperature
  • metallicity (and I cringe every time I hear this joke, because any time anyone says "metallicity" to a crowd of people, they follow it with, "To an astronomer, there's hydrogen, there's helium, and then there are metals," chuckle, chuckle, but...that's really the only way to say it)
These two properties lead to the determination of the stars' masses and radii.

So what is the problem?
The problem is that astronomers had been relying on the relationships (read: equations) used to analyze Sun-like stars to analyze low-mass stars

So what is the solution?
An awesome person (full disclosure: I say this because I know her so I know that the word "awesome" is objectively accurate) named Barbara Rojas-Ayala led the development of a brand-new, dwarf-specific way to determine metallicities and effective temperatures.

By observing these stars at near-infrared wavelengths, you can see signatures of three very important things: sodium, calcium, and H2O. The way sodium and calcium show themselves in the stars' light is correlated with how much metal is in the star (or, as I like to say, how "metal" the star is). Theirs is an empirical relationship--a relationship inferred from evidence, not necessarily explained by theory yet--found by observing at M-dwarfs with known metallicities, seeing what their sodium and calcium lines look like, and seeing how the two correspond. Then, with that knowledge programmed into your TI-83, you can go observe an M-dwarf's sodium and calcium and learn about its metallicity. The H2O observations combined with the metallicity allow calculation of the effective temperature of the star, again through an empirical relationship.

Are we there yet?
Not quite. But once you've got the metallicity and the effective temperature of the star, it's like stealing candy from a gas station to figure out the star's mass and radius and, from that, what you've all been waiting for: information about the planets themselves.

How different were the results from the new method?

Look at those nice red dots inside the nice habitable zone.
Credit: P. Muirhead
The old method said that planets around low-mass stars were about 200K hotter and twice as big as they actually were. When you use the new method and get cooler, smaller results, five new Earthesque candidates pop out.

Why should I care?
Well, obviously, because it's new information about the universe.

And also because it's cool for there to be potential habitable worlds around tiny stars.

And also because it shows that astronomical conclusions are not just data-dependent, but model-dependent. An assumption, or a known-but-the-best-we've-got fallacy, made at the beginning of analysis (in this case, that Sun-style knowledge can be applied to dwarf-style calculations) carries through all the steps it takes to get to a conclusion. And while your Algebra I teacher gave partial credit when you made a mistake and moved -x instead of +x to the other side of the equals sign but made no mistakes from there on out, it doesn't mean your final answer was right.



Philip S. Muirhead, Katherine Hamren, Everett Schlawin, Barbara Rojas-Ayala, Kevin R. Covey, & James P. Lloyd (2011). Near-Infrared Spectroscopy of Low-Mass Kepler Planet-Candidate Host Stars: Effective Temperatures, Metallicities, Masses and Radii Astrophysical Journal arXiv: 1109.1819v1  

Barbara Rojas-Ayala, Kevin Covey, Philip Muirhead & James Lloyrd (2010). Metal-Rich M-Dwarf Planet Hosts: Metallicities with K-Band Spectra Astrophysical Journal, 72- (113) DOI: arXiv:1007.4593v1

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Reader Comments (2)

Very interesting. When I hear stories like this in astronomy, I have a hard time understanding what model to believe - and if we should even invest in this new model, knowing that we're probably not right.

October 3, 2011 | Unregistered CommenterBrooke N.

Once empirical relationships can be turned into fundamental equations explained by theory, they can be trusted! We're often, though not always, on the right track.

October 3, 2011 | Unregistered CommenterSarah Scoles

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