Have you ever wondered how astronomers discover planets orbiting stars outside our Solar System? Agent Exoplanet takes you through the steps astronomers use when they study extrasolar planet transits.
In this activity, you will interactively measure the light from a star (choosing from one of six) which has a planet going around it. You will use the measurement of light to create a light curve (a plot of the changing brightness of the star and planet over time). Using this you will discover whether the star has an orbiting planet – this is the transit method used by astronomers across the World to discover and analyse new planets.
Not only will you measure the light from the star and planet, but you will also measure the brightness of 3 nearby stars, which should have a similar brightness to the candidate star. This is to ensure that you can “calibrate” your data by comparing it to stars that do not have planets going around them.
When you have gone through all of the images of your chosen star on Agent Exoplanet (ensuring the target is centred on the stars in order to collect data accurately), your data will be plotted on a light curve which will give you the size of the planet and its distance from its star. These are calculated from the ratio of light from the star compared to light during the transit. How to measure these values and the equations are provided on the final page of the activity along with your results.
This workshop, from Cardiff University, uses Agent Exoplanet to measure the sizes of planets as described above. As well as the transit method, this uses the radial velocity method to determine the mass of a planet based on observing the motion of the star. Just as a star’s gravitational pull causes a planet to move in its orbit, the gravitational pull of the planet on the star will also cause the star to move, though by a much smaller amount. The speed at which the star moves depends on the mass of the star, the mass of the planet, and the time it takes for the planet to orbit the star.
Combining these two methods provides more information about the planet. Using the planet’s size, from the transit method, and its radius, from the radial velocity method, it is possible to calculate its average density of a planet going around a distant star!
The workshop includes:
- A teacher guide [MS Word] ; [PDF]
- A teacher presentation [MS PowerPoint] ; [PDF]
- A teacher Excel spreadsheet for calculating planet masses (with answers) [MS Excel]
- A student worksheet [MS Word] ; [PDF]
- A student Excel spreadsheet for calculating planet masses [MS Excel]
- Radial Velocity Data for all six stars [MS Excel]
- Radial velocity analysis in LoggerPro [Data only] ; [Data and graphs] ; [Full analysis]
Radial Velocity data
The radial velocity data for the all six stars are available in Excel or LoggerPro format above. Individual data files (CSV format) are also available: [Qatar-1b] ; [CoRoT-2b] ; [GJ 1214b] ; [HAT 25Pb] ; [TrES 3b] ; [WASP 2b]
For example: the “wobbles” of the star Qatar 1 due to the presence of a planet called Qatar 1b are:
|Phase of orbit||0.201||0.832||0.565||0.235||0.661||0.058||0.758||0.465||0.167|
|Speed of wobble due to planet||-0.248||0.233||0.123||-0.185||0.143||-0.118||0.244||-0.033||-0.159|
Plotting this data in Excel or Logger Pro with speed on the y-axis and phase on the x-axis will produce a wave-like curve. The peak-to-peak amplitude of this curve (the height of the top peak to the bottom peak) gives the velocity of the star due to the presence of a planet (vs).
The mass of the planet (Mp) is calculated based on the mass of the star (Ms), the observed velocity of the star (vs), the period of the planet’s orbit (Pp) and Newton’s gravitational constant (G) using the following equation:
Mp = Ms2/3 vs Pp1/3 / (2πG)1/3
Extrasolar Planets Labs
The Extrasolar Planets Lab, from the Nebraska Astronomy Applet Project, includes two interactive applets, as well as more background information about the detection and characterisation of extrasolar planets: