by Ludwig Scheibe (TU Berlin), October 2024
The largest planets out there are massive gas giants. They are heavier than 80 times the mass of Earth, and larger than 8 times Earth’s diameter. With their size and mass, they closely resemble Jupiter and Saturn in out Solar System, and like these two, exo-giants are composed mostly of the gases hydrogen and helium, with heavier material like rock making up a core in the center.
But for these exo-gas giants, we have found a multitude of conditions that are quite different from our experiences with Jupiter and Saturn, and thus they present a challenge for our theories of planet formation.
Hot Jupiters
Two possible artist’s conceptions of what a gas giant exoplanet might look like. Left: “Cold Jupiter” at medium to far distance from its star, with very distinct and detailed atmospheric structures. Right: Hot Jupiter close to the star with broader, less distinct atmosphere bands and in the process of losing its atmosphere (see below). Credit: SPP 1992/Patricia Klein
The very first exoplanet around a Sun-like star, discovered in 1995, presented a surprise to planetary science: 51 Pegasi b is about half as heavy as Jupiter – so definitely a gas giant planet – but it has an orbital period of only 4 days, meaning it orbits its star at a distance of a mere 8 million kilometers. That is only one percent of Jupiter’s orbital radius, and just about one seventh of that of Mercury, our innermost planet. 51 Pegasi b immediately became the first in a class of planets unknown to us until then: The hot Jupiters.
This unusual bunch of planets are gas giants that are so close to their stars that they orbit it in less than about 10 days – as opposed to, for example, 87 days for Mercury, one year for Earth, or indeed 12 years for Jupiter. This proximity to their star means they receive a massive amount of stellar irradiation, which leads to them having outside temperatures of sometimes over 1000°C – hence the name.
Their discovery was surprising not only because we don’t have planets this close to our Sun, but also because according to most models of planet formation, such large planets cannot form so close to the star. The prevailing explanation is then that these planets formed further out from their star – similar to our own gas giants – and then moved consequently inwards through some migration mechanism.
Easier to find
We can find and study hot Jupiters particularly well with our standard methods. They are big and heavy, which is why they produce a big signal for the transit– and radial velocity method. Due to their small orbit period, the transit that reveals their existence returns often and it is possible to measure even several transits with relative short observation times. This is why in the early years of exoplanet exploration, we mainly found hot Jupiters. But it would be a wrong conclusion to think that these planets are particularly common. Ever since the Kepler-mission, we know there are far more super-Earths, so planets in size between Earth and Neptune, than hot Jupiters.
That does not change the fact that hot Jupiters are a fascinazing class of planets that is worth studying in detail. Their size and proximity to their star makes them ideal to study tides or the composition and dynamics of atmospheres. The methods can then be refined to use them for the smaller and harder to observe planet classes.
Loss of atmosphere
This infographic illustrates the effect that close proximity to a star can have on the extended atmosphere of a gas giant: It gets heated up, extends, and can even detach from the planet completely. Credit: SPP 1992 / Patricia Klein
One particular effect that can happen to these hot Jupiters is atmospheric loss: The atmosphere becomes so heated up that gas particles in the atmosphere begin to escape the planet’s gravity, leading to a cloud of gas trailing behind the planet on its orbit.
Infographic that illustrates atmospheric loss in close-in gas giants. Credit: SPP 1992 / Patricia Klein
Imaging far-out Jupiters directly
In rare cases, it is possible to image planets directly, albeit not in the visible but the infrared wavelength range. But that only really works if the planet is far from its host star so that you can see both objects separately. Furthermore, they have to be big and young (and therefore hot) planets, because only those give off enough thermal radiation to be imaged by us.
With this method we were successfully able to discover very young gas giants far away from their stars. often they are only a few million to a few hundred million years old – as opposed to our Solar System, which has been around for about 4.6 billion years. They give us valuable insight into how planets form and evolve early on, and are among the very rare cases when we can observe planets far from their stars.
Image of the binary star pair b Centauri A and B (top left), and their giant planet companion (bottom right). The planet orbits at about 500 times the distance of Earth to our Sun and has about ten times the mass of Jupiter. The third dot in the upper right is a background star. The picture was captured using the VLT in Chile. It was taken in the infrared wavelength range and converted to visible colors. Credit: ESO/Janson et al. under CC BY 4.0, with added labels