Monday, February 27, 2017

Snow Line and the Dwarf's Seven

I'm really sorry about the title. Not sorry enough not to use it, of course, but a little sorry.

So you may have heard about the recent discovery of a nearby solar system (a mere 39 light years away!) with seven planets all packed very close to the star (an M-dwarf). The discovery is significant because (a) some of the planets look to be rocky, Earth-sized, and in the habitable zone; (b) the relative nearness of the system makes it a prime target for further investigation; and (c) it's super rad. The occasion gives me the opportunity to explain a bit about how discoveries like this get made while waxing philosophical about the nature of astronomy itself. As a guy with an astronomy degree (I don't feel comfortable calling myself an astronomer) who (kind of) teaches an intro astronomy class, this is basically my job.

Conveniently, last week's discovery does an excellent job of illustrating three aspects of astronomy that I think set it apart from other sciences. (Or possibly my own confirmation bias leads me to see these aspects expressed, but let's leave that for another post.) These features are encapsulated in a kind of motto for astronomy that I've been using recently.

It goes like this: astronomy is the science of what you see when you look up. This sentiment conveys that astronomy is ancient and public, because for thousands of years, anyone could do astronomy just by turning their heads skyward and paying attention. Secondly, astronomy is bound (mostly) by sight, which is a limitation that forces astronomers to be both careful and creative. And finally, “up” is a pretty wide direction, and astronomy encompasses everything from the moon to other stars to the birth of the universe itself and anything else we find along the way.

All of this ties together into something truly remarkable. Astronomy has the power to transform points of light—the ever-present night sky that we rarely stop to consider deeply—into a story about exploding stars and merging galaxies and dark matter halos all under the spell of gravity in a dance that goes back billions of years and will probably continue for many orders of magnitude longer than it's lasted so far. And what's more, we have good reason to be confident in this story. How does astronomy manage to do this? Well, let's take a look at those seven newly discovered exoplanets.

While we've only known about exoplanets for a couple decades now, the study of planets more generally is, like the rest of astronomy, incredibly ancient. There are five planets visible to the naked eye (Mercury, Venus, Mars, Jupiter, Saturn) that have been known into antiquity. The first person to discover a new planet (Uranus) was William Herschel, using a telescope he constructed himself. Neptune followed, after Urbain le Verrier noticed that, after adding up all the known gravitational influences on Uranus, its calculated position on any given night was a little off from its observed position. He predicted that a planet farther out was gravitationally tugging on Uranus, so the astronomer Johann Gottfried Galle looked where le Verrier said to and found another new planet.

I'm giving this brief (and incomplete) history lesson because the fact of the sky always being up there makes astronomical discoveries collaborative and open. There's a parallel in last week's exoplanet discovery both in terms of that public nature and gravitational perturbations. Moreover, discovering new planets used to be a once in a generation kind of thing, but now we've discovered thousands of them and just found seven in one system. Astronomy is a gigantic, ever-expanding field; whenever we look somewhere new or look in a new way, we find new stuff.

So let's talk about TRAPPIST-1. While NASA had a big press conference about the discovery (and they were involved), this was a remarkably international effort, involving astronomers and telescopes from all over the world. Most exoplanets discovered so far have involved space telescopes because the atmosphere makes detecting subtle changes in a star's light curve difficult. A relatively cheap solution being used now is to image the same star many times either with multiple ground-based telescopes or the same scope repeatedly. This lets you produce a single, high quality light curve and means that anyone can get in on the exoplanet discovery game. With a small telescope that spends all its time looking at large patches of the sky, you can detect (and re-detect) the faint signatures of exoplanets. Once TRAPPIST and the other telescopes involved made those initial findings, NASA pointed the Spitzer Space Telescope at TRAPPIST-1 to confirm the discovery.

Okay, but how did these telescopes actually discover the seven exoplanets? This is where the central limitation of astronomy—sight is (just about) our only tool—leads to very creative solutions. The way that we transform TRAPPIST-1 from a point of light into a star with seven worlds is by performing high-precision photometry to construct a light curve of the star. A light curve is the change in a star's light over time. To get an accurate one, you need to get high quality images on short timescales. This runs counter to a very useful tactic in astronomy, which is to collect light from a source over a long period of time to produce a single, bright image. But if you do that, any deviations during that integration time get smeared out and missed.

To detect exoplanets, the deviations you're looking for are dips in the star's brightness at regular intervals. If your telescope, the star, and a planet happen to line up exactly, then every time the planet passes in front of the star from your perspective, the star gets a little bit dimmer. It's just like a solar eclipse here on Earth, except that these planets are much too far away from us to block out all the light of their parent star. Instead we see a tiny drop in brightness.

But these transits reveal a lot of information. First, the duration of the transit and the time between transits tell us how long the planet's year is. Combined with an educated guess about the star's mass (by taking its spectrum), we can figure out how strong gravity's pull on the planet is, and consequently the distance it needs to be from its star to complete an orbit in the observed time. The more massive the star, the faster a planet orbits at a given distance. Finally, the percentage of light blocked by the planet, combined with its distance, tell us how big the planet is compared to the star. Another educated guess about the star's size tells us the actual physical size of the planet.

So by looking very precisely at how a star twinkles, we can deduce the presence of a planet and make a reasonable guess as to how big it is and how close it is to the star. We can do this despite not actually being able to see the planet itself, which is much too small and dim next to its parent star to resolve. But I've been talking about one planet this whole time, and these astronomers discovered seven. You might think sussing out the details of seven different transits while also accounting for anything else that might mess up your photometry would be difficult, and you'd be right. The primary way the team identified seven different planets was through a statistical analysis of the transit times to come up with a chart that looks like this:

Credit: ESO/M. Gillon et al.
As a rule, planets don't share orbits. Doing so isn't stable. And each orbit has a definite period, and each period corresponds to an orbital speed, which tells you how long the transit should last. So if you identify a transit of a particular duration that repeats regularly, then you've found yourself a planet. If you see six or seven different regular transit times, you've found six or seven different planets.

There is a snag in all this, however, called TTVs—transit timing variations. That is, sometimes a transit happens earlier or later than expected. In this case, the variation could be up to half an hour. But it turns out this snag contains even more information, because this sounds an awful lot like the error le Verrier noticed in the orbit of Uranus. The planets weren’t where astronomers thought they would be given just the gravitational influence of the star, which means the planets—all extremely close to each other—are tugging on each other significantly.

Because so much is unknown about the system, the problem is much more complicated than the orbit of Uranus. Le Verrier was able to do a laborious calculation by hand using perturbation theory, but the complexity of TRAPPIST-1 require a slightly faster technique if you want to publish before the stars all die and we’re left in darkness. So instead the team constructed simulations of the system where they plug in the laws of physics and then vary the unknown orbital parameters to see what kind of planetary systems evolve that match the one they observed. In the end, they’re left with a set of possible masses that could produce the tugging required to account for the transit timing variations.

Even doing this produced a wide range of possible answers, which led to a great quote in the article: "The system clearly exists, and it is unlikely that we are observing it just before its catastrophic disruption, so it is most probably stable over a significant timescale." The relevance is that the system's existence is itself a piece of data, which means that as more observations are done, the assumed stability of the system can help to rule out orbital parameters that would produce an unstable system.

With those uncertainties understood, the team was able to estimate that most of the planets are in the neighborhood of Earth's mass. If you know the size and the mass, you also know the density. The worlds of TRAPPIST-1 are all rocky (high density) as opposed to gassy (low density). The proximity to the star itself is also important. If planets are too far out from their star—past the snow line—then water and other volatiles condense into ice. Far enough inside that line, however, and water can remain a liquid. Too close, and the liquid evaporates. These planets are all at the right distance to have liquid water.

An entire system of rocky, Earth-sized worlds warm enough to have liquid water—this is why everybody is so excited and why astronomers are going to keep watching these planets. The Kepler Space Telescope is currently looking at the system, and the James Webb Space Telescope will too when it launches. The relative nearness of the system to us means that it is fairly easy to observe. As new observations come in, we could learn about the planets' atmospheres—their density, composition, and variability—and whether they experience tidal heating and geological activity. Are these complex, intriguing worlds like the moons of Jupiter and Saturn or airless rocks scoured dry by the flares of their parent star? We just have to look up to find out.

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