Dr. Ben Davis, A Missouri State University astronomer coming from the New York University in Abu Dhabi, joins to describe his work observing hot Jupiters and a mysterious trend of inflating.
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Astro Brief is a podcast collaboration between KSMU, the Missouri Space Grant, and MSU's Department of Physics, Astronomy and Materials Science. Hosted by Dr. Mike Reed, Astro Brief focuses on astronomical events, the field of astronomy, and astronomy-related guests. It airs Thursdays at 9:45 am on KSMU.
Transcript
Dr. Mike Reed: Welcome to Astro Brief. I'm Mike Reed and this week we're talking with Dr. Ben Davis, formerly of New York University in Abu Dhabi and now a faculty here at MO State. Thanks for talking with us today.
Dr. Ben Davis: Pleasure to be here.
Dr. Reed: Would you begin by telling our listeners a bit about what a hot Jupiter is and what is puzzling about those planets?
Dr. Davis: A hot Jupiter is a gas giant planet similar in mass to Jupiter but orbiting extremely close to its star, often completing an orbit in just a few days compared to our Jupiter that orbits in about 12 years. Because these planets receive so much starlight, they are extraordinarily hot because they're so close to their host star. Now the puzzle we're looking at here is that many of them are also larger than we would expect, so we might say they're puffed up as compared to standard planetary models in cooling predict. We know that there's some process supplying heat or preventing heat from escaping but astronomers are still debating exactly what process is responsible.
Dr. Reed: How did you and your collaborators investigate that mystery?
Dr. Davis: We studied a sample of 328 hot Jupiters and used a statistical technique called causal discovery. So you might be used to finding correlations between data, but instead here we're choosing one physical explanation in advance and fitting it to the data. So in this method, which observed properties remain directly connected after the other properties have been taken into account. Considering each planet's mass, radius, orbital period and the temperature of the host star, that's the four parameters we studied for each hot Jupiter. And then as a reality check we applied the same method to 179 smaller super-Earths where we know in that example they're not puffed up, so we know there should be a strict or familiar mass-radius relationship. That gave us confidence that the method we're using provides physically meaningful patterns.
Dr. Reed: What did you discover about why hot Jupiters become inflated or puffed up?
Dr. Davis: What we found was that a hot Jupiter's radius remains directly connected to both its orbital period and the temperature of its star and this is even after accounting the other variables. There's this causal connection between these parameters we might say. Taking these two properties to determine how intensely the planet is irradiated. So our strongest conclusion here is that the inflation is regulated probably by the energy coming from the host star. Among the individual explanations we compared, thermal tides is probably the closest prediction or theory that matches our data. And in that scenario what you have is uneven stellar heating which produces atmospheric bulges and motions that ultimately deposit energy inside the planet. We can't necessarily rule out the other processes. These include heating from atmospheric circulation and electrical currents, but our results don't identify specifically one mechanism conclusively but there's strong evidence that the thermal tides is the closest match. And finally I just wanted to mention that it's very coincidental that you're interviewing me today because I just got an email from the astronomical journal that our paper was accepted for publication today.
Dr. Reed: Congratulations that's great. Let me kind of delve into that a little bit. One of the other possible processes could have simply been heat from formation being trapped inside and puffing up the planet is that correct?
Dr. Davis: Right. So there's more work to be done on this. This is just an early first step essentially that this fits well, the models, but the crucial next step is going to be to compare by what amount each planet is inflated not simply it's total radius, which is what we observe now. What we need to do in the future is to do a more detailed accounting for stellar energy, age, chemical composition and any observational biases. So what we want to do in the future is check whether orbital period still matters when two planets receive the same amount of stellar energy for example and if it does that would strengthen the case for thermal tides as we've indicated. And if the inflation depends mainly on the incoming energy, atmospheric circulation or electrical heating would become more likely as well. The broader lesson is that causal discovery methods can help turn large exoplanet catalogs into tests of the physics happening inside worlds where we obviously can't observe them directly.
Dr. Reed: What would astronomers need to do next to distinguish among those possibilities?
Dr. Davis: The exoplanet catalog is growing every day, I forget how many thousands we're up to now, but I think as observations get better and better, JWST is our greatest observatory right now to observe these exoplanets in detail if we can actually get these specific measurements of how much say, the intrinsic radius of these planets are compared to their puffed up radius, I think that would shed some light on what we're looking for.
Dr. Reed: Well thanks Dr. Davis our time is up I'd like to thank you for telling us about your interesting work on hot Jupiters.