Time and THYME: The Search for Exoplanets

By Abigail Dunnigan

Image by Felix Mittermeier. [CC0]

The unknown planet zips around its host star, undisturbed for millions of years. It continues to remain undisturbed, but for the first time we can see it. Dr. Andrew Mann and his team study these previously unknown planets, more specifically exoplanets found in telescope data from NASA’s Kepler, K2, and Transiting Exoplanet Survey Satellite (TESS) missions. Exoplanets are all planets outside of our solar system. Their goal is to look for planets in young star clusters, moving groups of stars (which move similarly through space), and star-formation regions. They hope that studying the statistical properties of stars at different ages will provide insight into planet evolution. Dr. Mann is an Associate Professor within the Department of Physics and Astronomy at UNC-Chapel Hill. He is part of the group that leads the Zodiacal Exoplanets in Time (ZEIT) Survey, as well as the TESS Hunt for Young and Maturing Exoplanets (THYME). The ZEIT Survey identifies faint or low-mass stars from K2 observations, while THYME uses data collected from bright or high-mass stars from TESS observations. “Zeit” is the German term for “time”, a clever wordplay for these projects.1 While they use different data, both the ZEIT Survey and THYME have the same overall goals: to study how statistical properties of planets evolve, thus shedding light on planet evolution. The primary tool for identifying exoplanets is through the transit method. Take a solar eclipse as an example. When the moon passes in front of the sun, it undergoes what is called a transit. While the moon is in transit, the areas on Earth in the path of the eclipse are darkened. This darknening occurs because the moon is blocking sunlight from reaching the Earth. The same principle applies as exoplanets far away from Earth pass in front of their host star. As the planet is in transit, it causes a small dip in the brightness of the star. The amount of Dr. Andrew Mann light detected at a given point in time is referred to as flux.

“The saying you go by is that you only know planets as well as their stars”

Other characteristics of the transiting planet can be calculated based off of the change in flux, but only if you know the properties of the host star. “The saying you go by is that you only know planets as well as their stars,”1 says Dr. Mann. To be able to measure the radius of the planet, “the drop [in light] is proportional to the ratio of the planet size to the star size. So you actually only measure the planet size as well as you measure the star size.”1 Dr. Mann and his team are currently focusing on THYME, which uses data from NASA’s TESS mission.1 The satellite collects

flux measurements of target stars every two minutes.2 To make sense of the data, they use statistical and computational methods to map it out into light curves. From there, they can determine properties of the host star and planets such as their radii, masses, and orbital periods. While many different analysis methods are used, light curves make it all happen. Each flux measurement is plotted as a function of time, and periodic dips in flux can be a sign of a transiting planet. However, that is not all that needs to be done. Stars are not static. Other factors, such as dark spots and flares, can cause either dips or spikes in the brightness of each star. In fact, the curves you see on a light curve (Figure 2) are most likely not the signals from the transiting planet.3 The star’s activity actually causes much more fluctuation in brightness than the planet does. In addition, younger planets usually orbit younger stars. Younger stars are more active, making it more challenging to find orbiting planets than if one was studying older stars. Despite this, younger stars are still a necessary component of studying planet evolution. The question remains then for Dr. Mann’s team as to how does one go about finding planets with all of this “astrophysical noise.” The answer is not simply by making better telescopes. More advanced telescopes would only measure the flux more precisely. The flux would not affect the brightness variations of the star, and the variation of the transiting planets would still be too small to detect just by looking at the light curve.1 A common mitigating technique that Dr. Mann and his team use is called the Notch Filter. It assumes that there is a transit in the light curve when there are small brightness variations because transiting planets slightly lower the brightness of the host star. To ensure that no transits are left out, a computer program uses an aggressive algorithm to run the data. This algorithm is more sensitive to flux changes. A downside to the Notch Filter is that because it uses such an aggressive algorithm, there is a lot of extra data that scientists like Dr. Mann have to sort out afterward. One way to discern between planets and “junk” data is to see if the variations in brightness occur periodically and obey orbital mechanics.1 The period of a planet is the amount of time it takes to travel one full rotation around its host star. Each transit Dr. Mann and his team view via satellite telescope happens once per period. The “notches,” or trapezoidal dips in the light curve, represent each transit that a planet takes (Figure 1).4 Figure 2. A closer look at the notch filter “dip” from planetary transit. One thing that benefits these projects is Dr. Mann’s research background. Prior to studying exoplanets in the ZEIT Survey and THYME, he primarily worked with stars. His background in studying stars aids in the search for exoplanets because he is more skilled in modeling host stars. In fact, the majority of researchers in the ZEIT survey and THYME have a background in stellar astrophysics.1 By having a stronger understanding of stellar astrophysics, Dr. Mann is better equipped to deal with current uncertainties; for example, there is still much that is unknown about young stars, as the underlying physics behind them is tricky and complicated. Dr. Mann is currently working on a paper connected to THYME. He and other researchers discovered two planets transiting a young, Sun-like star in the Ursa Major moving group. The host star in this system is bright, making it easier to analyze planet properties.3 Figure 2 shows the notch filter “dips” as each planet makes a transit. The blue and yellow arrows indicate where each transit is occurring.3 Notice how the yellow and blue arrows occur over equal periods of time. The arrows indicate how long each rotation around the host star takes. Since there is more space between the blue arrows, the planet they represent has a longer period than the planet represented by the yellow arrows. By using statistical and computational methods, Dr. Mann and his team were able to discover, characterize, and measure properties of the two exoplanets.

Figure 3. The light curve (top) indicates where each transit (bottom) is occurring.

Discoveries of exoplanets have already led to significant findings. For example, through his research, Dr. Mann has found that younger planets are statistically larger than their older counterparts. There are multiple theories for this, such as the hypothesis that younger planets are warmer and therefore expand in size.1 In the future, Dr. Mann intends on continuing with TESS to look for more young planets: “The big advantage of TESS is that the stars we find are statistically brighter. It’s better for this whole other regime of work we want to do called transmission spectroscopy.”1 Transmission spectroscopy goes further with studying the planets themselves, such as researching the atmospheres of these planets. THYME and the ZEIT survey have paved the way to conduct detailed studies of objects that are not even seen as

specks in the night sky.

References

1. Interview with Andrew Mann, PhD. 09/10/20 2. TESS Mission. https://heasarc.gsfc.nasa.gov/docs/tess/ (accessed September 25th, 2020). 3. Mann, A., Johnson, M., Vanderburg, A., Kraus, A., Rizzuto, A., Wood, M., Bush, J., Rockliffe, K., Newton, E., Latham, D., et al. astro-ph 2020, arXiv:2005.00047v2. 4. Mann, A. https://andrewwmann.com/ (accessed September 25th, 2020).