The above clip represents a true star in our sky that changes brightness periodically. What are possible explanations for this star's brightness changing? In thinking of a possible explanation, consider the following:

  1. Could it be possible for a star's intrinsic brightness to change?
  2. Could it be possible that something located between us and the star is affecting its brightness?

Below are three possible events that this star might be experiencing. Explain whether each of the following described events may or may not explain the star's changing brightness.

  1. This star is reaching the end of its lifetime. It has nearly finished burning all of its fuel, like a candle that has nearly burned through all of its wax.
  2. There is a darker smaller companion star, too faint to see, that is orbiting around the bright star.
  3. This video was recorded over a series of partly cloudy evenings at the photographer's location.

One way that we can describe the brightness of a star is with a graph that shows how the star brightness changes over time. We call this type of brightness versus time graph a light curve. Below are video animations of the three events described in the above question, along with an animated graph, or light curve, describing how the brightness might change over time.

Which of the following brightness versus time plot, or "light curve", could best explain the variable star that you saw at the beginning of this exercise?

The above light curve was generated with data from the Hubble Space Telescope. The drop in brightness occurs as a planet crosses in front of the star, and blocks some of the light. Inspecting the light curve, can you say anything about the size of the orbiting exoplanet? If you would like more explanation on the meaning of the x and y axis labels, click on them to see an explanation.

The exoplanet light curve graph above shows that the star brightness drops by about 1.7% when the planet passes in front of the star. Based on the amount of brightness drop, how much of the star is the planet likely covering up?

In our own solar system, Earth is big enough that it could block off 0.01% of the Sun's light, if it was covering up part of the Sun. Mercury would block off 0.001% of the Sun's light. Jupiter, the largest planet in our solar system, would block off 1% of the Sun's light. Does the exoplanet seen in the previous light curve most closely resemble:

If the drop in brightness repeats itself every 3.5 days, how long does it take for the planet to make one complete orbit around the star?

A video animation is shown that helps illustrate this fact.

If the planet takes 3.5 days to orbit its star (i.e., its period is 3.5 days), then any event that takes place once per orbit, like a transit event, takes place every 3.5 days as well. Therefore, we can use a light curve to determine a planet's period: any time the planet is in front of its sun (i.e., whenever that big dip in the light curve happens), we know that the planet has made another orbit. We can choose any point on the path of a planet's orbit and measure the amount of time it takes for a planet to circle back around to that point, but it is most convenient for us to choose the point in front of the planet's star.

The Earth takes 1 year to make a complete orbit around the Sun, our own home star. The exoplanet featured above takes only 3.5 days to make a complete orbit around its star. Is the featured exoplanet likely closer to its star, or further from its star, than the Earth is from the Sun?

Since the exoplanet takes only 3.5 days to make a complete orbital circle around the star, it must be making a much tighter orbit around the star, as compared to the Earth's orbit. For example, consider the animation below, which shows two possible orbits around a star. Note that the closer the planet is to the star, the smaller its orbit, and the shorter time it would take to make one complete orbit.

Based on your answer to the previous question, is this planet likely to be a comfortable place for intelligent life (similar to humans) to live, or a dangerous unpleasant environment?

Since the exoplanet must be very close to the star, it is likely being roasted by the star's heat. In such a situation, the planet is probably not a comfortable place for life to exist.



Allen, Richard Hinckley (1963) Star Names: Their Lore and Meaning, Dover Press.