Subject to Candidate to Planet: The Task of Zorro

If we’re confident that there is a real transit signal coming from a given star, we can apply for an allocation of time to use the Zorro speckle imager on the Gemini South telescope (which is a huge 8-metre telescope in Chile). This instrument takes lots of images of the star in quick succession, which allows us to “freeze out” the effects of the Earth’s atmosphere that causes light from stars to be distorted (similar to the distortion that causes stars to twinkle). This allows us to determine whether there are any nearby stars that might be contaminating our measurements. If a star were close enough that some of its light was “leaking” into our measurements of the candidate then we’d see a smaller transit depth than is actually the case. This would mean that we’d be underestimating the radius of the orbiting body which could mean that what we believe to be something the size of a planet could actually be the size of a star!

Below is a plot showing real data from the Zorro instrument on Gemini South for one of the most promising planet candidates from Planet Hunters NGTS (Subject 69654531 which you can read more about by clicking this link). The two different lines show data for two different wavelengths of light. The human eye typically sees light with any wavelength between 380 and 750 nanometres, but telescopes can focus on ranges centred around specific wavelengths with various filters. In this case, the telescope splits the beam of light (with a component called a dichroic) so that redder light (centred on 832 nm) goes to one camera while bluer light (centred on 562 nm) goes to the other. These two different measurements are shown in red and blue respectively on the plot below and allow us to obtain simultaneous two-colour imaging of the target.

The horizontal axis of this plot shows angular separation from the star in arcseconds, which is a unit used to measure the distance between points in the night sky (e.g. the Moon distance from one side of the moon to the other as you look at it from Earth is 1,900 arcseconds). The vertical axis is called “contrast” or “magnitude difference” and indicates how much dimmer the region around the star is compared to the star itself in units of apparent magnitude. What this plot shows is that between 0.1 arcsecs and 1.2 arcsecs, there aren’t any stars with an apparent magnitude within about 5 magnitudes of the target star’s brightness which lets us know that there probably isn’t a nearby star at these separations (although there could still be something really close in!). Using the results of these observations, we can be more confident that the transit we’re observing is “on target” (i.e. coming from the star we think it is) and we can have a more confidence in our estimates of the radius of our planet candidate as we know the levels of contamination aren’t significant. This gives us the necessary belief to push ahead with further follow-up observations which I will describe in the next part of this blog post series.