Virtual crowd of 300 attend Peter Gao's Neighborhood Lecture

Tuesday, May 24, 2022 

On April 28, 2022, Peter Gao kicked off our Neighborhood Lecture Series with a talk entitled, “Hot Jupiters, Super Puffs, and Lava Planets, Oh My!” 

The virtual talk drew around 300 attendees who tuned in from around the world to listen to Gao, an expert on planetary atmospheres, discuss the upcoming scientific discoveries waiting to be uncovered by the James Webb Space Telescope (JWST). 

According to Gao, the successful launch of JWST heralds a new age in the exploration of our Universe, and in particular the study of exoplanets. JWST's large aperture and wide and continuous wavelength coverage will allow for the detection and characterization of a host of atmospheric phenomena on exoplanets through observations of exoplanet transits, secondary eclipses, and phase curves, which will, in turn, revolutionize our understanding of exoplanet formation processes (and even habitability.)

During the talk, Gao discussed how astronomers make atmospheric observations of far-away planets and explained that a planet’s atmosphere can hold clues to a planet’s interior composition and its deep past.

He then took us on a tour of a type of planet called a “Hot Jupiter.” These planets are “extreme objects” according to Gao. They are huge and so close to their star that their atmospheres are at temperatures well in excess of 1000 K—temperatures hot enough to melt and vaporize rock. What does that mean? Rock clouds raining corundum (rubies!) and silicates.

Gao explains that one of the key things that several JWST programs aim to do is to determine the composition of these clouds, their particle sizes, how they impact the atmosphere, and how they impact our ability to observe the atmosphere.

He then introduced the audience to an extremely large, extremely low-density planet called a “Super Puff.” These planets are the focus of Gao’s own JWST program, which aims to observe these planets and determine how they might be “cheating” our density measurement system. One option includes massive rings that may be making the planet look bigger than it actually is. The other is that the planet may be swathed in planetary hazes. 

Finally, we explored Lava Worlds—rocky planets that are orbiting super close to their stars.

“The day side, we think, is hot enough to have an ocean of lava,” says Gao. “Above that ocean, we think there is an atmosphere of rock vapor.”

These planets differ from hot Jupiters because the rock vapor wouldn’t be mixed with things like hydrogen and helium—it would be pure rock vapor. But because these planets are likely tidally locked—one side of the planet is stuck facing the star—the other side of the planet is probably extremely cold. As the hot rocky atmosphere makes its way to that side of the planet, it likely recondenses into a thick crust of rock and ice. There are several JWST programs looking at the dynamics of this strange atmosphere to see if that is the case. 

If you liked this program, make sure to tune into our next Neighborhood Lecture, during which astrobiologist Michael Wong will explain the Science of Star Trek!

Highlights from the Q&A Session: 

Are super-Earths found in systems with hot Jupiters? And is there a typical system structure?

Peter Gao: There are definitely scientists out there looking for trends in system architecture. There are instances where you have small planets and large planets at the same time. It’s a bit of a wonder when you have a system with both because you expect the large planet to disrupt the smaller planet.

In general though, right now there is a debate because there appears to be a trend where the spacings between planets and the size of the planet are correlated. Why that is still being debated. When we have more planetary systems to play with, we will start to be able to understand these trends.

What’s in the core of a hot Jupiter? Are the elements and compositions different from our own Solar System?

Peter Gao: What’s in the core of a Hot Jupiter really comes down to what was in the protoplanetary disk of that system. 

A lot of our understanding of planets forms from our own system, and we think the core of Jupiter is made of primarily of various rock and ice species. They would have come together to form a core and then gas would have piled on top of that.

So something similar could be in the center of a hot Jupiter if they happened to form about the same distance from their star as our Jupiter and then migrated to their current, ridiculous location.

There is also the idea that the planet may have formed right there, in which case the core would be made of more rock than ice.

Regarding planets that might be conducive to life. Which chemical in a populated planet's atmosphere might be the most significant to detect?

Peter Gao: These questions are coming from people who’ve been reading papers!

Yeah, so. This is basically the field of biosignature detection. Originally we thought the detection of oxygen and ozone might be a sign of life. We also found that oxygen can come from other things, such as the breakdown of CO2 in the atmosphere due to stellar radiation.

Now the idea is that you need a combination of chemicals that should not exist if there were no life.

For example, oxygen and methane. Oxygen and methane tend to interact and form CO2, so if there’s something that’s keeping them apart—like life—or emitting a lot of oxygen and emitting a lot of methane at the same time, then perhaps that's a sign of life as we know it.

But I always caution people, that there is probably a lot of life as we don’t know it out there. 

Does JWST have polarimetric capabilities to measure the birefringence of chemicals like rutile (rock) in atmospheres?

Peter Gao:  JWST doesn’t have a polarimeter, I can say that much. Already I can say JWST will not be doing that kind of science, but some of the ground-based telescopes have been looking using polimetry due to the potential to study the polarization of scattered light from the host star due to these rock clouds.

For very young, directly imaged planets. Can you observe the cooling of these planets? And if so, can you see declining heat from short-lived radioactive isotopes?

Peter Gao: These young planets we can observe right now are giant gas planets, which are higher in helium, so in terms of radioactive nuclei, they aren’t really abundant enough to really see.

These are still cooling, not from radioactivity, but from the initial infalling of material into the planet. What we measure is that heat, that heat that is emitted from the planet as they cool. But that timescape is hundreds of millions of years, so we can’t really see them cooling, we just aren’t around long enough!

Is there a method to infer from planetary atmospheres to infer if planets have dynamic magnetic fields?

Peter Gao: Right now it’s hard to do that for rocky planets. For giant planets, like Hot Jupiters, there are the first inklings of detections of magnetic fields. Some of these planets are so hot that their atmospheres are ionized and their winds are tied to the magnetic fields. 

How can you measure the age of planets, and will you be emphasizing older planets?

Peter Gao: The age of planets comes from the age of their stars. The age of their stars comes from looking at their own compositions. One way is to look at lithium because stars destroy lithium in their cores. So, the amount of lithium can tell you how old the star is.

The other way is to look at associations because stars rarely form by themselves. They form among a whole cluster, and the cluster diffuses out. By using the star’s velocity you can check back to see if the star was in an association, and these clusters help you give an age because you know they all formed at the same time.

We don’t know too many young systems, we do prioritize young systems when we can. The result is we observe plenty of older systems just because they are more common.

What is going to replace JWST in the future?

Peter Gao: Funny things happen in astrophysics. We have the decadal surveys, which is where a lot of scientists come together to discuss what the field should do in the next ten years. The decadal survey came out last year, and they were trying to balance four different telescope concepts that would replace JWST.

The suggestions ranged from infrared and x-ray observers to essentially a bigger version of Hubble. A bigger version of Hubble would be in the UV and optical range. Now they’ve settled on that as the concept. It could be a telescope that has a mirror as big as JWST but is focused on observing UV.

Coming online sooner in parallel with JWST is the Nancy Grace Roman telescope, which is a microlensing and direct imaging survey telescope. Microlensing is another method for observing exoplanets where planets bend the light due to gravity. The geometry gets complicated, but you can figure out where the planet is and how massive it is based on the light bending.

The direct imaging part is that hopefully this telescope will allow us to look at reflected light from some of these more distant planets.  

If the JWST were orbiting another star, and it looked back towards Earth how likely would there be an assumption of life?

Peter Gao: First of all it would have to be a star where the Earth transits. If it’s transiting, Earth’s atmosphere is not really puffy enough for some of the observations we want to do. The puffier the atmosphere, the more chemical signatures we can pick out.

But let’s say the telescope is orbiting around Proxima Centauri or something really close. Then it might see signs of CO2, there might be some ozone bands, certainly some methane bands. By seeing these different chemicals and comparing them to our neighboring planets—like Venus, which would just look like CO2—they would be able to see something weird about our planet.

They’d also notice that it was the right distance from the star for liquid water to form, so they would be able to put two and two together and notice something different about Earth. 

Have we detected an extragalactic planet?

Peter Gao: Oh man, yes! Well…maybe.

A few years ago when they were observing a quasar or some very bright source, they saw a dip in the brightness that looked like it could be a planet transit. So that was a possibility of an extragalactic planet. 

If these hot Jupiters are raining rubies, what would the core be like?

Peter Gao: The ruby stuff is happening way up in the atmosphere. We think the core isn’t really touched by that. We think that some of the heat might be making its way to the core and inflating the planet, but I don’t think the ruby stuff is going to be connected.

What is the biggest thing that studying exoplanets has taught us about our own Solar System?

Peter Gao: It’s taught us about how unique we are. All of our ideas of planet formation started out by trying to explain our own Solar System. Rocky planets form closer in, giant planets form further out. Then we found Hot Jupiters and sub-Neptunes and these strings of rocky planets. So we find ourselves asking how unique is our solar system, how did that make life rise up?

The existence of Hot Jupiters has also put the idea of planet migration into our minds. So, one of the ways exoplanets have changed Solar System science is this idea, which we see in the Nice model and the Grand tack theory. What these hypotheses suggest is that the giant planets formed and then migrated around, resulting in throwing asteroids and Kuiper belt objects all over the place. Or, gas giants forming, moving in close to the star, and then moving out again.