Research

Cold Gas Giant transiting Exoplanets

Jupiter and Saturn have long been subjects of human fascination. Unfortunately, in terms of transiting exoplanets—which we observe indirectly when they pass on front of their host stars—these types of cold, long-period, gas giants are hard to discover and even harder to fully characterize. Yet their atmospheres hold precious clues that may help us better understand how planetary systems form and evolve in time.

I am particularly interested in finding these types of planets and characterizing their masses, orbits, and atmospheres. This can be especially difficult if transit events only happen once a year (or even less frequently). What this means is that every transit counts! I primarily focus on making mass measurements of these planets, coordinating transit follow up to catch the rare transits of these planets, and conducting simulations and prediction studies of how we can characterize their atmospheres. I have used an array of telescopes for these studies including the Discover Channel Telescope, the Spitzer Space Telescope, the Keck-I telescope, the Kepler satellite, the Transiting Exoplanet Survey Satellite and a bunch of small scopes situated around the world.

Radial Velocity Characterization

Many exoplanets have had their masses measured via the radial velocity (RV) method, sometimes called the "wobble" method. RV measurements for transiting exoplanets are especially powerful because we can combine the radius (from transit) and the mass (from RVs) to get a much better idea of the nature of the planet. With only one or the other, it's a little difficult to make headway into how the planet formed, what its interior is like, etc.

It turns out, the most long-period transiting planets don't have mass measurements. Long-term RV surveys are fairly common, but they don't usually focus on a sample of transiting exoplanets. To improve this situation, I have started long-term RV monitoring campaigns on the Keck-I telescope at W. M. Keck Observatory and the Automated Planet Finder Telescope (APF) at Lick Observatory.

Much of this work is ongoing or very soon to be published in the Giant Outer Transiting Exoplanet Mass (GOT 'EM) Survey. Here are the GOT 'EM papers that have come out so far:

• GOT 'EM I: Confirmation of an Eccentric, Cool Jupiter with an Interior Earth-sized Planet Orbiting Kepler-1514 (2021, AJ)

• GOT 'EM II: Discovery of a Failed Hot Jupiter on a 2.7 Year, Highly Eccentric Orbit (2021, AJ)

Transit Recovery

The majority of transiting exoplanets discovered to date have short orbital periods. This is largely a result of the observational bias of the transit method in favor of exoplanets that are very near to their host stars. The farther the exoplanet is from its star, the lower the chance that it will have just the right orbital inclination to transit from our point of view. However, staring at a large enough set of stars for a long enough amount of time (of course, I'm referring to the primary Kepler Mission) will lead to the discovery of a handful of long-period transiting exoplanets that more so resemble the planets in the Solar System in terms of length of year.

However, the troubles for long-period transiting exoplanets don't stop once they've been discovered. It turns out, that gravitational interactions with other planets or objects in their systems (if they exist) can cause substantial variations in the length of the exoplanets' years. For us, it makes it difficult to predict when future transits will occur and risky to use precious telescope time trying to observe a transit that might not happen when you expect. Because of this challenge, I conduct recovery efforts aimed at increasing the accuracy and precision of our predictions of future transit times for long-period transiting exoplanets. Two examples of these efforts have been published so far; take a look at what goes into this kind of work: Kepler-167e and Kepler-421b.

atmospheric refraction and transiting Exoplanets

Refraction is one of the most fundamental behaviors of light and it is indeed something we encounter in our every-day lives. In support of my PhD thesis, I conducted theoretical investigations of this process in terms of transit observations of long-period exoplanets. It turns out that before or after an exoplanet transits its host star, some of the stars light can be refracted into a distant observer’s line of sight by the planetary atmosphere. The end result of this is a secondary image of the host star in the exoplanet’s atmosphere—a.k.a. a mirage! This mirage causes the total flux of the unresolved star system to increase just a little. By modeling that brightness increase and understanding which types of planetary systems would clearly display such a phenomenon, I revealed one way in which refraction can help us learn about exoplanet atmospheres. It is also a method of discovery exoplanets that do not transit their host stars.

This result are published in the Astrophysical Journal, and you can read it on the arXiv here: https://arxiv.org/abs/1709.06991.

BTW: For this project, I developed a thorough ray tracing code that includes refraction in an exoplanet context. It’s call RETrO: Refraction in Exoplanet Transit Observations. I made the custom code available to anyone on github: https://github.com/pdalba/retro. If you find yourself in need of ray tracing simulations involving refraction, contact me!

solar system science from occultations

A planet is a planet is a planet, near or far. I firmly believe that the future of exoplanet science depends on our ability to incorporate the knowledge of decades of pure planetary science into our investigations. As such, I stay active in the field of planetary science.

Specifically, occultations of celestial bodies present extraordinary opportunities for Solar System science! I take advantage of natural and human-made occultations in nearly every aspect of my work (including the exoplanet stuff!). All of my work has utilized the truly amazing data sets taken by the historic Cassini Mission. Here are a few Solar-System-centered projects I am working on:

• Cassini solar occultations reveal Saturn's stratospheric thermal structure: As the Sun appears to set behind Saturn during a solar occultation (like the one in the picture above!), the Sun's shape and position are distorted due to atmospheric refraction. Anyone who has ever watched a sunset (or sunrise) over the horizon on Earth has seen this effect. As it turns out, this process captures the refractivity of Saturn's atmosphere, which is a critical parameter that is useful for understanding the thermal structure of the stratosphere. I am currently finishing up a project to extract atmospheric temperature from Saturn solar occultation observations. Keep an eye out for a paper (and some really cool refraction simulations) soon!

• Cassini radio occultations map the electrons in Titan's ionosphere: Radio occultations are a form of human-made occultations, where a spacecraft transmits a super-stable carrier signal to an Earth-based ground station while flying behind an object like Titan, Saturn's largest moon. The Cassini mission produced a phenomenal set of radio occultations observations of Titan's atmosphere. With the help of Prof. Paul Withers, I processed dozens of radio occultation observations to create many electron density profiles of Titan's ionosphere. This process essentially extracts a mHz signal at GHz frequencies, so it's quite extraordinary. Keep an eye out for a paper from Paul Withers and I describing this process and our results for Titan.

• Cassini observes Saturn as a transiting exoplanet: I used more Cassini observations to explore what we could learn if we found and characterized a Saturn-twin exoplanet. As it turns out, despite being very cold and cloudy, the atmosphere of a Saturn-like exoplanet is exceptionally amenable characterization! Current and future observatories such as the Hubble Space Telescope and the James Webb Space Telescope will have the capabilities to probe these atmospheres to learn about methane content and even amazing disequilibrium processes such as photochemistry! This work was published in the Astrophysical Journal in October 2015, and it also received some press attention. Take a look at the articles listed in the Media Coverage section of my website.