Habitable Extrasolar

0.1 Detecting Extrasolar Planets We can detect extrasolar planets using a method similar to the method we used to measure binary . Except, now we are looking at one of our stars being much bigger and brighter than the other, so the dip in the light curve is very small, so space-based observations are needed to detect the . Below are two examples of a light curves measured by kepler.

How well does the data fit the red line shown?

In both these figures, the y-axis plots the relative flux, where 1 is the brightness of the , and the dip indicates how much of the light is blocked by the planet. What fraction of the star’s light do each of the planet’s block?

1 We can find the radius of the planet compared to the star based on this change. Find the radius of Kepler-67 using the formula:

R 2 ∆f = planet Rstar where ∆f is the dip in brightness you measured, Rplanet is the radius of the planet, and Rstar is the radius of the star. The star this planet orbits is G9V star, so has a radius of about .778 solar radii, or 5.42×105 km.

Earth has a radius of 6,731 km. Is Kepler 67 a similar size to ?

We can also estimate the semi-major axis based on the period of rotation. Using Kepler’s Third Law, P 2M = a3 where P is the in a is the semi-major axis in AU, and M is the of the star, or .865 solar .

This star is a little cooler than our , but not by much. Do you think this planet is habitable?

2 Detected Extrasolar Planets The info for a few other extra-solar planets is given below, along with some infor- mation about the planets in our for comparison. Do you think any of these planets could be habitable?

Semimajor Orbital Orbital Mass Planet Mass Radius Temperature Axis Period Eccentric- of Star Name (MJup) (RJup) of Star (K) (AU) (Days) ity (MSun) Mercury 0.0002 0.03 0.4 88 0.21 1 5778 Earth 0.003 0.09 1.0 365 0.0167 1 5778 0.0003 0.05 1.5 687 0.09 1 5778 1 1 5.2 4,270 0.05 1 5778 Saturn 0.3 0.8 9.6 10,756 0.06 1 5778 HD 10180 0.067 - 1.42 602 0.00 1.06 5911 g HD 99109 0.50 - 1.11 439 0.09 0.94 5262 b HD 28185 5.8 1 1.02 379 0.05 0.99 5656 b HD 73534 1.07 1 3.02 1,770 0.07 1.17 4884 b HD 183263 3.57 1 1.49 626 0.36 1.12 5936 b 55 Cnc 0.173 - 0.77 261 0.32 0.9 5196 f

3 What do you think make an extra solar planet more difficult to observe?

We define the habitable zone as the range of distances from the central star in which liquid water might exist on the surface of the planet if the planet has a dense enough atmosphere. Will the habitable zone be the same around every star? Why or why not?

The habitable zones for the planets in the previous tables are given below. They where chosen as they spend at least some of their time within the habitable zones of their stars. Is this a good indication that they could all have life? Are there other properties that might cause issues? If you could only choose one planet to send a probe to to search for extra terrestrial life, which would you choose? The paths of these planets are shown on the last page, for further information.

4 Planet Name Inner Habitable Zone (AU) Outer Habitable Zone (AU) Earth 0.95 1.4 HD 10180 g 1.13 1.94 HD 99109 b 0.82 1.43 HD 28185 b 0.95 1.65 HD 73534 b 2.14 3.78 HD 183263 b 1.15 1.97 55 Cnc f 0.77 1.35

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