A misconception about extrasolar planets

A couple of weeks ago in the introductory “Astro 101” class I work in, the instructor and I confirmed that many students hold a certain misconception. I was, still am, pretty excited about this little discovery in astronomy education. If my conversations over the following few days had turned out differently, I probably would be writing it for publication in the Astronomy Education Review. Maybe I still will. But for now, here’s my story.

Our search for life in the Universe and the flood of results from the Kepler Mission have made the discovery of extrasolar planets an exciting and relevant topic for introductory “Astro 101” courses and presentations to the general public.  Instructors, students, presenters and audiences latch onto “the transit method” of detection because it is so intuitive: when an extrasolar planet passes between us and its star, the planet temporarily blocks some star light and we detect a dip in the brightness of the star. The period and shape of the dips in the record of the star’s brightness encode the characteristics of the planet.

When an extrasolar planet passes between us and its star (when it "transits" the star) we detect a dip in the brightness of the star. (Kepler/NASA image)

Our students do a nice 50-minute, hands-on lab about how to decode these “light curves” which I hope to share at the ASP 2011 conference (#ASP2011 on Twitter) in July. In a class following this lab, the instructor posed the following think-pair-share clicker question. We wanted to assess if the students remembered that the size of the dip is proportional to the area of the star blocked by the planet’s disk, which scales as the square of the diameters:

Clicker question to assess the students' grasp of the transit method of detecting extrasolar planets.

The bars in this histogram record the number of students who chose (from left to right) A to E:

Students' responses for (left to right) choices A to E to extrasolar planets clicker question.

About 60% of the class chose answers (C and E) with a 1% drop in brightness, the correct drop, and about 40% chose answers B and D with a 10% drop. This second group didn’t remember the “proportional to area” property. So, not stunning results, certainly a good candidate for pairing and sharing.

The misconception

What is stunning, though, and the source of my excitement, is that 97% of the class feels you see a black spot moving across the star. Which is not true! We only detect the drop in the brightness of the star. We can’t even see the disk of the star, let alone a tiny black spot!

Okay, okay before you jump to the students’ defence, let me (with the help of my great CAPER Team colleagues) jump to the students’ defence:

  1. The question says, “…by observing it pass in front of the distant star.” Of course the students are going to say we see a dark spot – that’s what we just told them! Perhaps I should be worried about the 3% who didn’t read the question properly.
  2. The question is vague about what we mean by “size.” Diameter? Area? Volume? Mass? “The star’s diameter is 10 times bigger than the planet’s diameter” is a much better question stem.
  3. My colleague Aaron Price points out
  4. Astronomers may not see a “dot” crossing the star right now, but they can see something comparable. Through speckle imaging, radial topography and optical interferometry we have been able to see starspots for decades. CHARA’s recent direct observations of a disk of dust moving across epsilon Aurigae shows what is being done right now in interferometric direct imaging. I predict within 10 years we’ll have our first direct image of a “dot” in transit across another star.

  5. Aaron, Kendra Sibbernsen and I all agree that the word “see” in “What would you see?” is too vague. The question I wanted to ask should have used “observe” or “detect”. Kendra suggested we write “A) a dark spot visibly passing in front of the star” and perhaps following up the question with this one to poke explicitly at the potential misconception:

With current technology, can astronomers resolve the dark spot of an extrasolar planet on the disk of a star when it is in transit? (T/F)

Was there a misconception?

Did the students reveal a misconception about transiting extrasolar planets. Nope, not at all. It’s not like they took the information we gave them, mixed it with their own preconceived notions and produced an incorrect explanation. Instead, they answered with the information they’d been given.

A teachable moment

It seems that we’re not being careful enough in how we present the phenomenon of transiting extrasolar planets. But as it turns out, this is a teachable moment about creating models to help us visualize something (currently) beyond our reach. We observe variations in the brightness of the star. We then create a model in our mind’s eye — a large, bright disk for the star and a small, dark disk for the planet — that helps us explain the observations.

This is a very nice model, in fact, because it can be extended to explain other, more subtle aspects of transiting extrasolar planets, like a theoretical bump, not dip, in the brightness, when the planet is passing behind the star and we  see detect extra starlight reflected off the planet. The models also explains these beautiful Rossiter-McLaughlin wiggles in the star’s radial velocity (Doppler shift) curve as the extrasolar planet blocks first the side of the star spinning towards us and then the side spinning away from us.

These wiggles in the radial velocity curve are caused by the Rossiter-McLaughlin effect (from Winn, Johnson et al. 2006, ApJL)

Want to help?

If you’re teaching astronomy, you can help us by asking them this version, written by Kendra, and letting me know what happens.

An extrasolar planet passes in front of its star as seen from the Earth. The star’s diameter is 10 times bigger than the planet’s diameter. What do astronomers observe when this happens?

A)  a dark spot visibly passing across the disk of the star
B)  a 10% dip in the brightness of the star
C)  a 1% dip in the brightness of the star
D) A and B
E) A and C

In conclusion

I don’t think this qualifies as a misconception, not like the belief that the seasons are caused by changes in the distance between the Earth and the Sun. We’re just need to be more careful when we teach our students about extrasolar planets. And in more-carefully explaining the dips in the light curve, we have an opportunity to discuss the advantages and disadvantages of using models to visualize phenomena beyond our current abilities. That’s a win-win situation.

Thanks to my CAPER Team colleagues Aaron, Kendra and Donna Governor for the thoughtful conversations and the many #astro101 tweeps womanastronomer, erinleeryan, uoftastro, jossives, shanilv and more who were excited for me, and then patient with me, as I figured this out.

About Peter Newbury

Find me on Twitter @polarisdotca

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4 Responses to A misconception about extrasolar planets

  1. Erin says:

    Ha, we had a chat in the car (me and the extragalactic other half) on Rossiter-McLaughlin effect and how you can use it to determine if the planet is orbiting prograde or retrograde (I made the mistake of mentioning that and extrasolar planets in the introduction of my dissertation which I made him read last week).

    But I guess I can see how the kids would extrapolate black dot = observing it, especially given the diagrams that you have for the examples with black dots. They’re taking it as observing it pass= direct detection, not that observe could include more indirect methods that infer the presence of a planet. I think the rephrased question has the same issue to with the first sentence reading “as seen from Earth.” Seen by whom? It’s possible that once again they’ll equate “seen” to directly detected via imaging.

    Sound like a very good spot though to discuss or reinforce the ideas of direct vs indirect detections in astronomy.

    • Thanks for the comment, Erin. Sounds like you have interesting chats in the car. Perhaps you’re right about the revised version of the question. “As seen from Earth…” does suggest we see the planet. And “…from Earth” could exclude Kepler. Maybe we could leave out that sentence, altogether. After all, we’re declaring there is a dip in the brightness of the star, which suggests a transiting extrasolar planet, which suggests the planet travels in front of the star. What about “An extrasolar planet passes directly between us and its star….”? That’s a statement of what happened, not what we see.

  2. Erin says:

    I think the rephrasing of what happens works much better than the saying what’s seen. Because to many of them seen is a direct measurement. Although thinking about it, I think that maybe it would be good to run with a course-long emphasis on direct vs indirect observations (and use the theme for clickers and pair/share: is this way of finding planets/dark matter/etc a direct or indirect observation?) . We already show students in our courses galactic rotation curves as evidence for dark matter, and as a lab instructor I found myself doing a lot of explaining about direct vs indirect methods in astronomy after those lectures because they didn’t feel comfortable annoying the prof in a gigantic ~200 person lecture.

    And yes we have interesting car conversations. Someone hates when I talk about compositional gradients in protoplanetary disks with my hands when I’m driving ;-)

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