Category: astro 101

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.

Persian New Year

Ali Narimani

Ali Narimani is a astronomy graduate student at UBC. Ali facilitates the labs we run in our introductory “Astro 101” course. While developing an activity about the motion of the Sun across the sky, we were trying to figure out why students should care about predicting the equinox. Ali excitedly said, “…because of Persian New Year!” He’s a great TA who cares about helping students learn and about his Persian culture. He emailed this story of Persian New Year to all the astronomers in our Department on Sunday, March 20, 2010, the day of the Vernal Equinox. With his permission, I’m posting it here.

[Update March 19, 2012: This post was originally published March 20, 2010. In 2012, equinox occurs on March 20 at 05:14 UT]

Persian New Year is celebrated at spring equinox of the northern hemisphere. Today at 4:20pm, we will start our new year which is also the first year of a new decade. Our new year does not start at a certain time of the day each year, but we should tune ourselves for the moment of equinox. (which is on March 20th at around 4:20pm, this year)


Traditionally, member of the family sit around a table called “Haft-Sin”, meaning “Seven S”, some minutes before the start of the new year and pray for a healthy, wealthy, and happy year. The table is called Seven-S because there should be 7 different objects on the table starting with the consonant S:

  1. Sprouts (Sabzeh) : is a symbol of *rebirth*
  2. Apple (Sib) : is a symbol of *beauty*
  3. Garlic (Sir) : is a symbol of *health*
  4. Coin (Sekke) : is a symbol of *wealth*
  5. Silverberry (Senjed) , is a symbol of* love*
  6. Vinegar (Serke) : is a symbol of age and *patience *
  7. A very Sophisticated Persian Cuisine (Samanou)

I have attached a picture of this table:

The table called "Haft-Sin", meaning "Seven S"

Right after the new year time, the usual part of celebration is hug and kiss, and the most fun part for the kids is to receive their new year gifts. New year holiday lasts for 13 days and during this period members of the big family (including all Aunts & Uncles) go to each other’s place for a visit. Younger members should visit elders first. The fun part of these short visits is that everyone wears nice new clothes, and again the kids receive some gift from their aunts and uncles. (the definition of Kid is a little vague in here since my grandmother still likes to give some “new year money”, kept in Quran for a couple of days for holiness, to my 65 year old father as new year gift).


I have attached two traditional Persian dances:




Ali Narimani
Department of Astronomy and Physics
University of British Columbia

Another day of agile teaching

The prof I’m working with in our introductory #astro101 class at UBC surprised me today. I thought he was sabotaging a teachable moment when in fact, he pulled one of the most “agile” moves he’s made yet. Here’s the story:

Today is March 21, 2011, the first full day of Spring. The vernal equinox occurred yesterday, March 20 at 4:21 PDT. The instructor, let’s call him H, started today’s class with a clicker question:

The correct answer is A) but I fully expected a bunch of students to vote B), confusing the “going North” and “going South” for the Sun’s motion along the ecliptic.

The students thought, then voted. H looked at the results and said (I’m paraphrasing from memory),

The correct answer is A. 70% of you said that…

Oh, no, I thought to myself. He just gave away the answer and the success rate – only 70%, not terrific – and totally short-circuited the teachable moment that comes via peer instruction.

That thought took about 1 second, of course, so it was all over by the time H continued with

…Very few of you said B, C, or D and 30% said E. Let me show you one slide and then I’ll come back to the super moon.

The "super Moon" as seen from Vancouver. (Credit @gmarkham, used with permission.)

You see, there was another event this past weekend. The full Moon occurred near perigee, the point in the Moon’s orbit around the Earth when it is closest. This means we had a full Moon, closer than usual, so it appeared bigger. Super, even. Oh, and it was.

So, here I was, getting alarmed that H was missing the opportunity for the students who voted A) to convince the students who voted B) to change their answers. But that’s not what happened at all. Hardly anyone voted B. They either knew the right answer A) or were more interested in the astronomy-in-real-life super Moon event. And H agilely, er, with great agility, confirmed the correct answer and followed up with an something 30% of the students cared about. He talked about the full Moon, how it was 14% bigger and 29% brighter. Not twice as big – don’t believe everything you hear on TV. That’s slightly bigger and closer than usual but not much. And no, the super Moon did not cause the earthquake in Japan.

Wow. I was impressed. He had the whole thing planned out but tailored his response based on theirs. Cool.

What about you? What teaching have you done, witnessed or experienced that shows agility?