Worksheet: Weighing Worlds

This worksheet goes with the Wobble Fitter activity.

NamePartnerMy Transit Detective star

Part A: The wobble

1. In the barycenter waltz, who moved in the bigger circle: the lighter person or the heavier person? Why?

2. When the whistling phone swung toward you, the pitch went ______ . When it swung away, the pitch went ______ .

3. A star's light does the same thing. When the star moves toward us, its spectral lines shift toward the ______ end of the spectrum. When it moves away, they shift toward the ______ end.

4. Jupiter makes the Sun wobble at about 12.5 m/s. Earth makes it wobble at about 0.09 m/s. Which planet would aliens find first by watching the Sun's wobble?

Part B: Weigh the planets

Open the Wobble Fitter. Do 51 Pegasi as a class, then your Transit Detective planet. If your Transit Detective star was C (LHS 3844), do pi Mensae instead. If it was F (the impostor), do WASP-62.

For each planet, adjust the period, the wobble speed K, and the left/right slider until the fit meter says Excellent.

If you switched planets, you didn’t measure your new planet’s radius: take it from the class results table or the Wobble Fitter’s ‘Use the published radius’ button.

Planet	Radius of this planet (Earth radii)	Density (g/cm³)	Made of?
51 Peg b	not measured	—	—
My planet:
A classmate's planet:

1. 51 Peg b doesn't transit. How did you find its period without any transits?

2. For the same period, what happens to the planet's mass when K doubles?

Part C: What is it made of?

Type your Transit Detective radius into the Wobble Fitter to place your planet on the mass–radius diagram. For reference: water is 1.0 g/cm³, Jupiter 1.3, rock 3–5.5 (Earth is 5.5), iron about 8–11.

3. Where does your planet sit on the diagram? Which Solar System planet is it most like?

4. WASP-18 b and WASP-62 b are almost the same size. Compare their masses and densities. Which one would float in a (very large) bathtub?

5. Why do we need both a transit and a wobble to know what a planet is made of?

Part D: Why haven't we found Earth 2.0?

Open "Sun (seen by aliens)" in the Wobble Fitter and try to find Earth.

6. Could you fit Earth's wobble? What makes it so hard?

7. Fill in which planets each method finds most easily:

Method	Easiest planets to find	Hardest planets to find
Transits
Radial velocity (wobble)

8. Given your answers above, are the planets we've found so far a fair sample of all the planets in our galaxy? Explain.

Part E: Exit ticket

Two planets have the same radius and orbit similar stars with similar periods. Planet X makes its star wobble at 1,800 m/s. Planet Y makes its star wobble at 60 m/s. Which planet is denser, and about how many times denser?