There are two main methods for discovering exoplanets (planets around other stars). The first is the radial velocity technique which detects the a wobble in the stars radial velocity because of the gravitational effect of the planet. The second is the transit technique which detects the faint dimming of the star as the planet passes in front of it. Both techniques are challenging because they require the correct alignment and the signal is small, so very precise instruments are needed.
Laboratory Tools
In this lab we will be making use of two simulators created by the astronomy department at the University of Nebraska-Lincoln. Note that in both these simulators the scale on the y-axis changes so pay attention to it. The simulators are:
- The Exoplanet Radial Velocity Simulator – this simulator show the radial velocity technique for identifying exoplanets. The upper left box shows the planet in orbit and the direction from which we are viewing it. The upper right panel shows a plot of the radial velocity of the star versus the phase which is just the fraction of the orbit the planet has gone through. You can check boxes to show the theoretical curve and to show simulated measurements. The amount of noise and number of data points can be controlled if the box for show simulated measurements is checked. On the bottom left there is a box Animation Controls that lets you start and stop the animation which just makes the planet go around. In the bottom middle you have boxes for System Orientation and Star Properties. These controls can change the angle the system is being viewed from and the mass of the star. The bottom right has a panel Presets that has drop down menu of preset properties for the planet and a panel Planet Properties to set those properties yourself.
- The Exoplanet Transit Simulator – this simulator shows the transit technique for discovering exoplanets. The upper left box shows a picture of the star and the planet transiting it. The upper right box shows a plot of the normalized flux from the star, with 1.0 being the flux when there is no planet transiting. You can check boxes to show the theoretical curve and to show simulated measurements. The amount of noise and number of data points can be controlled if the box for show simulated measurements is checked. In the middle on the left side is a box, Presets, which lets you choose preset values for the planet’s properties from a dropdown menu. Below it are the Planet Properties which you set individually. In the middle on the bottom is a box for Star Properties which lets you set the star’s mass and in the bottom right there is a box System Orientation and Phase which lets you set the inclination and longitude of the system.
Assignment
Let us start with the Exoplanet Radial Velocity Simulator. Start with only showing the theoretical curve which is easier to follow and with preset A that has the star’s mass as 1.0 solar mass, the planet’s mass as 1.0 Jupiter mass, the semimajor axis as 1.0 AU and an eccentricity 0.0. Record the maximum and minimum radial velocity on the plot. Now change the mass of the star from 0.2 to 2.0 solar masses. How does this effect the radial velocity? Return the star’s mass to 1.0 and now change the planet’s mass, what happens? Return the planet’s mass to 1.0. Adjust the semimajor axis from 0.01 to 10.0, what is the maximum radial velocities. What is the maximum radial velocities in these cases? Return the semimajor axis to 1.0 and change the eccentricity to 1.0, What happens to the radial velocity plot? Return the eccentricity to 0.0 and change the inclination between 0 and 180. How does the inclination effect the observation? Change the longitude, this should have no effect when the eccentricity is 0.0. Change the eccentricity to 1.0 and now adjust the longitude again. What effect does this have on the radial velocity?
Now let’s switch to the Exoplanet Transit Simulator. Again start with only showing the theoretical curve which is easier to follow and with preset A that has the star’s mass as 1.0 solar mass, the planet’s mass as 1.0 Jupiter mass, the semimajor axis as 1.0 AU and an eccentricity 0.0. What is the lowest value for the normalized flux? How long does the eclipse (transit) last? Now change the planet’s mass, planet’s radius, semimajor axis and the eccentricty. How does each effect the flux and length of the eclipse? Reset to the original values. Now change the star’s mass from 0.5 to 2.0 solar masses. What effect does this have on the flux and length of the eclipse? Finally adjust the inclination, don’t use the slider you need to make small changes. Change the inclination to 90.1, 90.2, 90.25 and 90.3. What effect does inclination have on observing the planet’s transit?
Questions
- What combination of properties leads to the easiest detections of extra solar planets using the radial velocity technique?
- What combination of properties leads to the easiest detections of extra solar planets using the transit technique?
- For the transit technique what disadvantage does a more massive star cause for detecting an exoplanet? What advantage do you gain?
