by Ludwig Scheibe (TU Berlin), October 2024
On the grand size scale between massive gas giants and smaller super-Earths, we find a class of medium-sized planets: Worlds with masses roughly between ten and 60 times the mass of Earth, or sizes of 3 to 6 times Earth’s radius. This includes the size of Uranus and Neptune in our Solar System – about 4 Earth radii and 15 Earth masses – and that is why this class is often called exo-Neptunes. What do we know about this link between small rocky worlds and large Jupiter-likes?
Artist’s rendering of TOI-1231 b. With a size of about 3.7 Earth radii and a mass 15 Earth masses, it is only slightly smaller and lighter than Neptune in our Solar System. Credit: NASA/JPL-Caltech
What are they made of?
Like super-Earths, exo-Neptunes can have a broad range of compositions. The similarly-sized planets from our Solar System, Uranus and Neptune, have a core of rock and iron, surrounded by an envelope dominated by materials like water, methane and ammonia, and a thick hydrogen-helium-dominated atmosphere around that. Because the envelope-forming compounds like water are often found frozen in the outer Solar System, where Uranus and Neptune reside and have formed, the planets are sometimes called “ice-giant”, though the materials in their deep interior are not frozen and rather in a state of fluid or plasma. These are states of matter under extreme pressures and temperatures which we are not familiar with from our usual surroundings.
It stands to reason that exo-Neptunes are built similarly, but probably with different proportions of the components. The only quantity that can usually help us constrain a planet’s interior makeup is its average density, and that covers a broad rangefor Neptune-sized exoplanets, suggesting their composition varies from planet to planet. Some resemble the comparably “fluffy” nature of gas giants like Saturn, while others with high densities presumably have larger amounts of rocks and ices.
Hot Neptune Desert
Planets close to their star are relatively easy to detect with the standard methods of transit and radial velocity, and we have found numerous large gas planets, as well as small rocky worlds on close orbits to their stars. Given that, we would expect to find also close-in, so called “hot”, Neptune-sized planets. We have, however, only found very few of these, a phenomenon sometimes called the “Hot Neptune Desert“.
A proposed explanation is that the intense radiation of the star strips away the outer lighter envelopes of gas- and ice giants, leaving behind just the smaller core of the planet, which we can detect as small planets. The close-in large gas giants we measured, so-called hot Jupiters, might have migrated inwards only later in their lifetime, when the stars typically emit less UV radiation and so their gas envelopes would not have been stripped.
Artist’s rendition of ultra-hot Neptune LTT 9779 b. Credit: Ricardo Ramirez
One of the few inhabitants of this “desert” is LTT 9779 b, a planet of 30 Earth masses and 4.6 Earth radii which orbits its star in under a day and has an estimated outer temperature of 1700°C. Scientists estimate that the stellar irradiation is in the process of stripping this planet’s atmosphere, as shown in the artist’s illustration above.