Icy Protectors: Why It’s So Difficult to Find Planets Beyond the Snow Line
Most of the exoplanets we know of are either very large or orbit very close to their stars; like hot-Jupiters. But what about smaller planets that orbit very far from their stars? They’re rather a rarity. Why?
This JWST photo shows Uranus and its rings; Source: ESA
It seems when it comes to planets, ice giants are often ignored. Despite their fascinating colors and far orbits that gives them their names, we don’t know a lot about the two planets farthest from the Sun: Uranus and Neptune.
It’s been nearly 40 years since a spacecraft first and last visited these two icy planets. I already wrote an article about why it’s so important to go back exploring them, and one of the reasons is: learning about Uranus and Neptune tells us a lot about exoplanets.
Among the most common sizes of exoplanets we know of are those intermediate in size between Earth and Neptune — based on their composition, they are either called super-Earths or mini-Neptunes. Sure, it’s entirely likely that this “most common” declaration is the result of observational bias; after all, despite modern technologies it’s still a difficult feat to actually discover exoplanets.
Even so, we don’t have super-Earths or mini-Neptunes in the Solar System. (My theory is that Earth would have become a super-Earth or mini-Neptune if it hadn’t been for the collision that formed the Moon, but maybe I’m completely wrong.)
However, it’s much easier to estimate how a super-Earth might be composed based on all the things we know from decades of exploration of the terrestrial planets in the Solar System (and not to forget that the very planet we stand on has a lot to reveal).
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| This graph shows the most common types of exoplanets based on 2017 data. You can see here that mini-Neptunes and Neptune-sized planets are among the most common; Source: Universe Today |
What about mini-Neptunes? As noted above, all we know about the ice giants stems from data collected by Voyager 2 and careful observations with modern telescopes. We just don’t know enough about them. And that’s why we have to go back. Another reason for an urgent return is the very fact that Uranus and Neptune exist — if they exist, then ice giants also exist in exoplanetary systems.
But what exactly constitutes an ice giant that we can say “this exoplanet is an ice giant”? Ice giants like Uranus and Neptune are similar to Jupiter and Saturn in that they don’t have solid surfaces. However, while Jupiter is composed primarily of hydrogen and helium, ice giants are made of heavier volatiles such as water, methane, and ammonia.
At the distances that they are from the Sun, these elements are ices, existing as supercritical fluids in the interiors of the planets.
One mystery that surrounds the ice giants is how they form. Our basic theory for planetary formation is that young stars have a swirling disk of gas and dust from which the planets develop. In this theory, planets beyond 20 AU (1 AU = average distance Earth-Sun) have a difficult time developing — yet this is exactly where we find Uranus and Neptune in the present-day Solar System.
Astronomers think that the ice giants formed much closer to the Sun but then migrated outward.
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| The hot-Neptune desert refers to the lack of Neptune-sized planets close to their stars; Source: EurekAlert |
This is very important; while we know of a lot of hot-Jupiters in exoplanetary systems, there are rarely any hot-Neptunes. This has been dubbed the hot-Neptune desert. Most Neptune-sized planets are therefore found as warm-Neptunes. Some of the warmest of them are losing their atmospheres, indicating that they can’t be stable so close to their stars.
So we have to look for actual ice giants; Neptunes that lie beyond the snow line. The snow line is where water can only exist in a frozen or supercritical state. When the Solar System formed, the snow line was probably located where the present-day asteroid belt is found, 2.7 AU from the Sun.
Here comes the challenge: the farther from the star a planet is, the more difficult it is to find. The two main ways by which exoplanets are discovered is either when a planet transits the star and the light is temporarily dimmed by a small percentage or by detecting the “wobble” a planet induces on the star due to its gravity.
However, with ice giants neither works efficiently; all because of their far orbits. Being so far from their stars, we’d have to wait for decades or even centuries just to witness a transit. Seeing a transiting ice giant would be a lucky coincidence. And the distance also means that the planet barely has any detectable gravitational effect on the star.
Another method has to do—gravitational microlensing. This is when an object’s gravity distorts the light of a more distant object, essentially working like a magnification lens. But microlensing still depends on a lucky alignment between two objects. Yet it has helped us discover exoplanets in the past.
Gravitational microlensing has led to the discovery of Jupiter-like planets in orbits similar to Jupiter’s, and it also led to the discovery of ice giants!
In 2014, astronomers discovered a planet 25’000 light-years from Earth. That’s already extremely far away, and it turns out to be very similar to Uranus. The planet is more massive yet located at roughly the same distance as Uranus is from the Sun.
A planet that is a bit closer has the mouthful designation MOA-2012-BLG-006L b, discovered in 2017. It orbits at a distance of 10 AU from its star, which is about 1 AU farther than Saturn is from the Sun. Though the planet isn’t an ice giant and bigger than Jupiter, it takes 46 years to finish one orbit!
However, not all planets far away from their stars are discovered via gravitational microlensing. For example, Kepler-421 b transited its star — a lucky case here. The planet’s year is equal to two Earth years, or about one Martian year. Kepler-421 b sits beyond the star’s snow line and has about the size of Uranus.
Kepler-421 b sets the record as the transiting planet with the longest orbital period! But we obviously know of exoplanets with much longer orbital periods. These are particularly revealed via direct imaging. What is that?
Well, it’s as straightforward as its name suggests: you photograph the planet. Now, that’s easier said than done. Imaging an exoplanet is difficult for various reasons:
- Planets close to their stars go down in the star’s sheer glare
- Since planets are much smaller and only reflect light, they don’t shine as brightly as their stellar parents
- And of course you can imagine how difficult it is to photograph something as far away as an exoplanet…
However, astronomers have managed to take photos of planets — and when you take a look at the planets in question, a pattern emerges: the planets usually orbit young stars and are extremely far away from their stars.
In this case, extremely far away isn’t even far enough: extremely, extremely, extremely, extremely far away.
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| Direct image of GU Piscium b and its red dwarf; Source: Wikipedia |
For example, the planet GU Piscium b orbits a red dwarf 160 light-years from Earth. The planet has about nine times the mass of Jupiter and resides 2000 AU from its star! That means one year lasts 163’000 years. In comparison, Voyager 1 is currently about 160 AU from Earth!
But while GU Piscium b is so far away from its star, it’s not an ice cold world like Uranus and Neptune. The planet has a temperature double that on the surface of Venus.
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| COCONUTS-2b orbits its star at a separation of 7500 AU; Source: Wikipedia |
But well, the extremities of GU Piscium b are nothing compared to COCONUTS-2b. This planet with a mass of six Jupiters orbits its red dwarf at a distance of 7500 AU — one year takes over one million years. You really need to have a long life span in order to celebrate a birthday in this system.
We could discuss all day such distant planets. For example, in the HR 8799 we know of four planets, the closest of which still takes 45 years to finish one orbit, the farthest takes 460 years…
That’s all great, yes, but none of these are actual ice giants!
Our knowledge of practical examples of ice giants may be scarce, but all the microlensing has taught scientists one thing: that Neptune-mass planets are the most common type of planet to form at vast distances from their stars. A 2016 study presented this, including that cold Neptune-like planets are much more likely than Jupiter-like planets in Jupiter-like orbits.
This tells us two things: we may be very lucky having Jupiter in our Solar System, and it’s hightime we pay more attention to Uranus and Neptune! We have to study them in order to learn more about exoplanetary ice giants, which in turn will reveal more about the origin of the Solar System.
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