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The graph plots the un-calibrated signal minus the average signal from the instrument. When a planet passes in front of the star (making a transit across the star), the total light output drops accordingly. This causes the larger observed dips in the graph.

Note #1: If desirable, the plot may be printed so that the data may be measured more accurately. Simply click on the graph and then print the resulting web page.

Note #2: If no transits are observable in the data, then go beack to the previous page and select a different star.

I. Calculating Orbital Information from the Observational Data

A. Period of the Exoplanet

From the graph above, calculate the average time between transits of the planet across the star face. (Find the day of the first and last transit and divide by the number of time intervals between these transits.) Then enter this period in days in the formula below.

B. Distance of the Exoplanet from Its Parent Star

The third law of planetary motion derived by Johannes Kepler (and modified by Isaac Newton) connects the orbital period of a planet in our solar system, the mass of the Sun and the planet's average distance from the Sun.

Astronomers have been able to estimate the mass of a star if it is a main sequence star (on the H-R diagram) and if its spectral type is known. See the table.

Locate the spectral type for this star and read off its mass. Then enter this number in the appropriate empty box below.

Kepler's third law can be written as:

where

• p is the orbital period of the planet in units of years,
• M is the mass of the star in units of solar masses,
• a is the average distance the planet is from the star in Astronomical Units.

Notes on the Photometric Observations

• If no significant dips in the signal are observable, then several other possibilities may be at work.
  • There might not be a planet orbiting this star.
  • The planet may be too small or the star too far away for instruments to detect the effect of the planet's transits.
  • The planet mgith be too far away from the star to have made a transit during the length of time the instrument was collecting data.
  • Maybe no planet passes directly in front of this star, even if it has one or more planets orbiting it.

Notes on Kepler's Third Law

• While Kepler's third law was derived from data for planets in our solar system it has been found to provide a good description of a planets orbit about any star, if the mass of that planet is small compared to the mass of its star. Essentially all exoplanets discovered to date fit this criteria, and the Earth-size ones which the Kepler Mission will hunt for will definitely match this assumption.
• The easiest units for mass in this equation are solar masses, where the mass of the Sun is equal to 1 solar mass
• The average distance between a planet and its parent star is the semi-major axis of the planet's orbit about the star and should be given in Astronomical Units (AU).

Hints (Reminders)

• Use the appropriate buttons. Do not press the "Enter" key.
• Enter or change numbers only in the appropriate boxes.
• Complete all "calculations" in order from the top of the page to the bottom.

More Info

• Return to the main Finding Exoplanets page to learn more about the search for planets outside of our solar system.