Evening Landscape with Exomoons

I often work out my thoughts on the topics we discuss here while taking long walks. I try to get in five miles a day but more often it’s about three. In any case, these long, reflective walks identify me as the neighborhood eccentric, an identity that is confirmed by the things I write about. What’s interesting about that is that so many people have a genuine interest in the stars and how we might get there. Some of the best questions I’ve ever had have been from people whose interest is casual but persistent, and one good question usually leads to another.

Hence I wasn’t surprised on yesterday’s walk to find myself talking with a neighbor about exomoons and why we study them. After all, we have a Solar System in which moons are commonplace. Isn’t it perfectly obvious that different solar systems would have planets with moons?

The answer is yes, but it also follows that things that seem perfectly obvious still have to be confirmed. But let’s unpack it a bit more than that. We’re familiar with our own system’s configuration, in which moons of astrobiological interest are orbiting gas giants a long way from the inner system. But we know from our exoplanet work that large planets like these can exist in warmer places. Thus the notion of habitable moons around gas giants, or perhaps double planets in the habitable zone, something like a larger version of Pluto and Charon in a comfortable orbit.

Popular films like Avatar keep the exomoon theme in front of the public, whose interest is understandable. After all, could anything be more exotic than a warm gas giant orbited by something a bit like the Earth? From an astrobiological perspective, the thought of Europa or Titan analogs in warm orbits is thrilling, a reminder that life may have gained many footholds in the galaxy. The Hunt for Exomoons with Kepler project is all about figuring out the occurrence rate of large moons so we can learn whether such moons are common.

The other aspect of exomoon detection has to do with increasing our expertise. It wasn’t so long ago that we had yet to detect our first exoplanet. Now we’re delving into planetary atmospheres and working out the orbital dynamics of multi-planet systems. An exomoon detection would be a major proof of concept, demonstrating the growth of our skills. It would also begin to build an exomoon catalog that will help us understand how important exomoons may be to planetary habitability. How big a role does our own moon play in keeping our planet habitable?

There’s also plenty to learn about how planetary systems form in the first place. We now think our Moon formed in a massive collision (the Big Whack) with a Mars-sized object in the early days of our planet’s history. How likely an event is this, and how often does it happen in other Solar Systems? We still have a lot to learn about how the satellite systems around various planets emerge, especially when we consider the wild variety of moons we see in our Solar System. Building the exomoon catalog will help answer these questions.

The Joys of Beta Pictoris b

I hadn’t planned to get into exomoons today, but serendipity struck. After yesterday’s conversation I ran across Phil Plait’s latest essay for Slate. The popular astronomer and science popularizer (author of Death from the Skies! and, of course, Bad Astronomy), now explains that because of an unusual alignment beginning in 2017, we may be able to detect an exomoon, if there is one, around the planet Beta Pictoris b.

We’re dealing with a system far different from our own. Some 60 light years away, the star Beta Pictoris is more massive than the Sun and a mere infant, at 25 million years old, compared to our own star (around 4.5 billion years). This is a solar system in formation. Moreover, it has been under intensive study since scientists realized it was surrounded by a large circumstellar disk. The planet Beta Pictoris b was first imaged in 2003, a world more massive than Jupiter that orbits its host every 20 years. You can see its movement in the time-spaced images below.

Image: Infrared images of the planet β Pictoris b obtained in 2003 (a), 2009 (b) and 2010 (c), showing the planet’s movement in an orbital plane that is nearly edge-on as seen from Earth. The host star is in the central part, but its light has been suppressed to show the fainter planet. The white dots in b and c denote previous positions of the planet. Faint blobs are optical effects. It is not possible to tell from these images whether the planet is orbiting towards or away from us, but {Ignas] Snellen and colleagues’ spectroscopic observations clearly indicate that the planet is currently in a part of its orbit where it is moving towards us. Credit: ESO.

Note in the description above that the planet’s orbital plane is close to edge-on from our perspective. It’s not close enough to make a transit possible, but what Plait talks about is

the next best thing. Drawing on a paper by Jason Wang (UC-Berkeley) and colleagues, Plait explains that the region around the planet called its Hill Sphere will pass in front of the star from our perspective. The Hill Sphere is the area around an astronomical body in which its gravity dominates. In other words, within the Hill Sphere, a moon could be retained by the planet.

Nobody explains such concepts as well as Phil Plait, so I’ll give him the floor here, drawing directly from his essay:

The size of the sphere depends on the mass of the planet, the mass of the star, and the distance between them. For example, the Earth’s Hill sphere reaches out to about 1.5 million kilometers. The Moon, orbiting 380,000 km away, is well inside that, so its motion is mostly influenced by the Earth (some people like to say the Moon orbits the Sun more than it does the Earth, but those people are wrong). Weirdly, Pluto’s Hill sphere is much larger than Earth’s, but that’s because it’s so far from the Sun that an object can orbit Pluto from farther away and still be heavily influenced by it.

What emerges with regard to Beta Pictoris b is that its Hill Sphere is 160 million kilometers in radius. We get no transit of the star by the planet itself, but by August of 2017, the planet will be at its closest approach to the star and the Hill Sphere region will transit. We’ll be able to look for debris or exomoons. A large moon passing in front of the star would be the first entry in the exomoon catalog.

But even if we get no exomoon detection, bear in mind that we may make other interesting observations. This young planet is still being born, and it may well contain a circumplanetary disk of its own, or even a ring system that is the residue of planet formation. “The transit of β Pic b’s Hill sphere,” Wang et al. write, “should be our best chance in the near future to investigate young circumplanetary material.” We’ll also learn a lot more about how Beta Pictoris b perturbs the circumstellar disk, a window into early solar system formation.

All this is good material for my next walk and the conversations sure to follow. The paper is Wang et al., “The Orbit and Transit Prospects for β Pictoris b constrained with One Milliarcsecond Astrometry,” accepted at the Astrophysical Journal (preprint).