Tag: stars

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.

Leveraging Public Outreach

Every winter for the last 7 years, my Department has put on a science show for the general public, following the tradition celebrated by physicist Michael Faraday (1791-1867).

Orion (Credit & Copyright: Matthew Spinelli)

This year’s theme was “The Physics of Light and Colour”. We did demos about lasers, solar power, optical illusions, twinkling stars, this cool Cubee device developed at UBC. For my 10 minutes, I talked about colour and temperature – how we usually portray cold as blue but to astronomers, blue = hot. I encouraged everyone to go home and look for the bright, coloured stars in the constellation Orion.  (I projected Star Walk from my iPad thru the theatre projector to show everyone where to look. Oh my! Fantastic!)

We had an amazing turn-out: nearly 350 people, at least half of them kids!

Before and after the show, I set up a four Galileoscopes on tripods outside the building. Sure, it was to attract people and tell them they were in the right place. But really, it was so I could get a bunch of kids looking through my telescopes. It was the middle of the day so we looked at signs, cars, people, a statue and a Caterpillar excavator. And they were all upside-down! What’s up with that, kids?

When it was over and I had a few quiet moments to take down the telescopes, I thought about the best parts of the afternoon, and came to a new realization about why I do “sidewalk astronomy”. From least to most important:

3. I love looking at stuff through my telescope. I can still remember the first time I saw Saturn’s rings. This is one of those defining moments for many budding astronomers, like watching Neil Armstrong take his one small step is the defining moment for almost every astronaut who’s ever talked about why they became an astronaut.

2. I love helping kids see stuff through my telescope. Typically, kids run up, grab the scope, jam their eyes near the eyepiece. Talk talk talk blab blab squeal. And then the image snaps into focus. “blab blab squ-. . . . . . oooooooh!” I love that moment! (Tip: if you’re aiming at something really bright like the Moon, often you see the image forming on the kid’s cheek and you can gently maneuver their head to get the bright image on their pupil.)

1. This is the new one for me. It was so rewarding talking to the kids’ parents. They’re already interested enough that they’ve brought their kids to your event. If you can get them enthusiastic and excited, they’ll carry on the conversation and discoveries at home. And they’ll be the ones who experience that magical moment when their kids see Saturn for the first time.

Unfortunately, it means I give up the golden moments with the kids — and that’s hard to do because it’s like a drug. But it leverages my enthusiasm and excitement. And knowledge: knowledge of astronomy and knowledge of how to teach astronomy.

Still, here I am, writing about how much I enjoyed talking with the parents. Maybe I’ve found a new drug…

The birth of a clicker question

It’s easy to come up with poor clicker questions, ones that merely test who has memorized X, Y, or Z from the previous slide. Or questions where there is no way to figure out the answer: either you’ve got it or you don’t.

Good clicker questions, on the other hand, take some time to create. Sure, you might stumble onto a good one every now and then, and it gets easier as you do it more. But it’s really gratifying when you put in the time, and it works. Here’s my story.

The constellation Orion
The constellation Orion(APOD 2008 October 15)

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