Researchers are working hard to make models of the atmosphere of exoplanets. By examining the emission spectra of planets without life, they can improve finding planets with life. But the studies of hot Jupiters – hot gas giants – have also proved to be valuable for other purposes. Adjusting climate models to planets that are 10,000°C shows much more clearly which effects are the most important one. A new study to understand the abnormally large radii of inflated hot gas giants might thus help us to predict the future climate of Earth.
Exactly 1 year ago, NASA sent a piece of equipment weighing a little over 6,000 kg on a journey of 1.5 million kilometres. Via the 18 1.3-metre-wide gold-coated beryllium mirrors comprising a mirror 6.5 metres in diameter, astronomers hope to gather information about the universe’s first stars and galaxies, but at least as important is detailed atmospheric characterisation of potentially habitable exoplanets – Earth-like planets – where life might ultimately be found. For astronomers, the mission is about much more than finding extraterrestrial life.
“There is no doubt that characterisation and understanding of the atmospheres of exoplanets will give us an important basis for finding life in the universe, but maybe even more important, it will help us solve some of the longstanding questions about the atmospheric phenomena on these planets. In our recent study, we built a model of hot and ultra-hot Jupiters to try to understand the mechanism that inflates these planets. Combined with future data from the telescope, this will help us to understand not only these distant planets but also the dynamics on Earth, including climate dynamics,” explains Aaron David Schneider, PhD student, Centre for ExoLife Sciences, Niels Bohr Institute at the University of Copenhagen.
Still a mystery
Many hot Jupiters are much larger than expected. They exhibit larger radii and lower densities than expected from existing models developed by researchers. This problem of understanding the dynamics of these planets, which can reach temperatures of several thousand degrees Celsius – is called the inflation problem. And the answer to why the planets inflate has been far from easy for researchers to get.
“There are several possible reasons why they are larger than expected. In my research, I tried to determine whether one reason could be that there is some atmospheric dynamics that function as a heat engine in pushing down energy received in the upper atmosphere above, pushing down by these atmospheric dynamics towards deeper layers, so that you have hot deeper layers that would then puff up the planet’s atmosphere,” says Aaron David Schneider.
Earlier this year, the researchers presented the results of a calculation that compared the amount of atmospheric expansion for two exoplanets with very different amounts of atmospheric inflation. One could envision that different rotation rates would give rise to different large-scale downward flows of the energy shining on the planet from the star it orbits and hence give rise to different amounts of inflation of the two planets’ atmospheres. This effect has some similarities to how the Sun shines energy onto the top of the Earth’s atmosphere and the Earth’s rotation moves it around. However, the new study shows that such large-scale downward motions cannot explain the differences in the observed expansion of the atmospheres.
“We find that our climate model does not give this puffing up. We do not see this heat engine in our in our climate model. So, there is no mechanism just by considering the dynamics, and the winds and the irradiation on the planet and so on. If we use the new model, our planets seem to cool down instead, even if you start them out hot. They just cool down over time and shrink so to speak. So, this is still a mystery that we have to solve,” explains Aaron David Schneider.
Can adjust our own climate models
Even though the solution to the mystery is still unclear, the researchers do have clues. One of the most promising mechanisms to explain the inflation problem is microturbulence, since the atmospheric dynamics simulations performed of the hot Jupiters only consider winds on a large scale.
“They do not include small friction winds, which could also transport energy. I think that this is the most promising explanation, because you could have a cascade of these small-scale turbulence cascades that transport energy downwards. So, I think that this could be a reason for inflation,” explains another main author, Uffe Gråe Jørgensen, Professor in Astrophysics and Planetary Science, at Niels Bohr Institute at the University of Copenhagen.
Even though the hot Jupiter WASP-76b, on which the new simulations are based, is 640 light-years from Earth, the researchers use climate models that were originally developed for Earth, including a 3D wind system and temperature model – adapted to work on exoplanets.
“When you make climate models on the Earth, you have a lot of free and unknown parameters in your models, which can then be fixed by balloons that fly up and measure, so you can correct what is not right. And so they can somehow disregard some of the real physics. But then one can also say that the disadvantage is that the model then only works for the climate we have right now and for the Earth we have right now,” says Uffe Gråe Jørgensen
Much can be learned by testing climate models developed for Earth with something as extreme as a hot Jupiter.
“We will not really see much of the things that could be a little bit wrong with the models down here if you have a 0.6°C rise in temperature, but if the temperature rises several degrees, then it may be important. If you go out to the Jupiters at 10,000°C instead, then you will see this effect much more clearly right now,” says Uffe Gråe Jørgensen.
Not just smooth sailing
Thus, if the models fail on these faraway planets, we might ask ourselves why we are sure that we can use these models for our Earth. Therefore, the new models of hot Jupiters can thus potentially be used in a near future on Earth for weather predictions and for predicting the future of the climate, but here and now the models are applied to the exoplanets, where they also serve to answer other important questions.
“So how do the signals – the emission spectra from these planets – change if there is biology present on one of these planets? To answer this question, we need to know what they look without life. We have two types of models. So, we have what we call one-dimensional, where we consider the planet as one average. And then we see what kind of spectrum comes out of that and how that is affected. And such models, once they work, can be computed pretty rapidly,” explains Uffe Gråe Jørgensen.
However, the cost of being able to do that relatively rapidly on a computer is that researchers have to make simplifications. One simplification previously made is the weather.
“Previously we had to assume that there is no wind, no mountains and or anything like this. Just smooth sailing. And that is probably far from the truth on these ultra-hot planets,” says Uffe Gråe Jørgensen.
With the new and complex models, simulations take longer, but precision is hopefully greater.
“We hope it will help us to become better in determining what the spectrum would look like when we look from the Earth at these planets. If there are different conditions and maybe all kinds of physical and chemical conditions, but it may also be biological conditions, so by simulating an environment that can help us to learn how life affects the atmosphere of our planet and of other planets to eventually spot if there is life out there,” concludes Aaron David Schneider.