Icy Guardians: The Challenges of Discovering Planets Beyond the Snow Line
Written on
Chapter 1: The Rarity of Distant Exoplanets
In the realm of exoplanets, most known examples tend to be either quite large or orbit their stars at close distances, such as the hot-Jupiters. But what about the smaller planets that reside far from their stars? These distant worlds are notably scarce. Why is that?
Not a member? Access the full story for free here!
Ice giants often remain overlooked in discussions about planetary types. Despite their striking colors and remote orbits, our understanding of the two farthest planets from the Sun—Uranus and Neptune—remains limited. Nearly four decades have passed since a spacecraft last explored these intriguing icy giants. I previously discussed the significance of revisiting them; studying Uranus and Neptune could unlock valuable insights into exoplanets.
Learn more about ice giants here!
Among the most frequently encountered sizes of exoplanets are those that fall between Earth and Neptune—often classified as super-Earths or mini-Neptunes. This prevalent categorization may stem from an observational bias; despite advancements in technology, the discovery of exoplanets is still a formidable challenge.
Interestingly, our Solar System lacks super-Earths or mini-Neptunes. (I hypothesize that Earth could have evolved into one of these types if not for the impact event that formed the Moon, though I might be mistaken.)
Estimating the composition of a super-Earth is more straightforward due to decades of exploration of terrestrial planets within our Solar System, not to mention the insights our own planet can provide.
Section 1.1: Understanding Mini-Neptunes
As previously mentioned, our knowledge of ice giants is primarily derived from data gathered by Voyager 2 and meticulous observations from modern telescopes. Our understanding is still insufficient, underscoring the need for further exploration. Another crucial reason for a return mission is that the existence of Uranus and Neptune implies that ice giants likely exist in exoplanetary systems as well.
What exactly defines an ice giant? Like Jupiter and Saturn, ice giants lack solid surfaces. However, their composition is distinct, consisting of heavier volatiles like water, methane, and ammonia. At the distances these planets orbit the Sun, these substances exist as ices or supercritical fluids in their interiors.
A significant enigma surrounding ice giants is their formation process. The prevailing theory suggests that young stars are surrounded by rotating disks of gas and dust, which eventually coalesce into planets. According to this theory, planets beyond 20 AU (with 1 AU being the average distance from Earth to the Sun) face challenges in forming—yet this is precisely where we find Uranus and Neptune today.
Astronomers propose that these ice giants originated much closer to the Sun and subsequently migrated outward.
Section 1.2: The Hot-Neptune Desert
This is significant because, while numerous hot-Jupiters have been discovered in exoplanetary systems, hot-Neptunes are notably absent. This phenomenon has been termed the "hot-Neptune desert." Most Neptune-sized planets are found in warmer regions, and some of the hottest are shedding their atmospheres, indicating instability in close proximity to their stars.
To locate actual ice giants—Neptunes positioned beyond the snow line—is essential. The snow line is defined as the region where water can only exist in a frozen or supercritical state. When our Solar System formed, the snow line likely resided where the current asteroid belt exists, approximately 2.7 AU from the Sun.
Herein lies the challenge: the greater the distance from a star, the more difficult it becomes to detect a planet. The primary methods for discovering exoplanets involve observing transits (when a planet passes in front of its star, causing a temporary dimming of light) or detecting the gravitational "wobble" induced by a planet's presence.
However, these techniques are inefficient for ice giants due to their distant orbits. Observing a transiting ice giant would be exceptionally rare, requiring decades or even centuries for a single transit to occur. Additionally, their distance limits their gravitational influence on their stars.
Gravitational microlensing presents another discovery method. This process occurs when an object's gravity bends the light from a more distant source, effectively acting as a magnifying lens. However, microlensing still requires precise alignment between objects, making it a rare event. Nevertheless, it has previously facilitated the discovery of exoplanets.
In 2014, astronomers identified a planet 25 light-years away that bears similarities to Uranus. Although more massive, it orbits at a distance comparable to that of Uranus from the Sun.
Another notable discovery, designated MOA-2012-BLG-006L b, was made in 2017. This planet orbits its star at a distance of 10 AU, slightly farther than Saturn's distance from the Sun. Although not an ice giant, it is larger than Jupiter and takes an impressive 46 years to complete one orbit.
Interestingly, not all distant planets are discovered through microlensing. For instance, Kepler-421 b managed to transit its star—a fortunate occurrence. This planet's year spans two Earth years (about one Martian year) and resides beyond its star's snow line, with a size comparable to Uranus.
Kepler-421 b holds the record for the transiting planet with the longest orbital period! However, many exoplanets exhibit even longer orbital cycles, often revealed through direct imaging.
Chapter 2: The Quest for Direct Imaging
Direct imaging, as the name suggests, involves capturing photographs of planets. However, achieving this is far from simple due to various challenges:
- Planets close to their stars are often obscured by their stellar glare.
- Planets are significantly smaller and only reflect light, making them less bright than their stars.
- The sheer distance of exoplanets complicates imaging efforts.
Despite these obstacles, astronomers have successfully captured images of some planets, revealing a common pattern: these planets typically orbit young stars and are located at substantial distances from them.
For instance, GU Piscium b orbits a red dwarf star 160 light-years from Earth. This planet is about nine times more massive than Jupiter and orbits at an astonishing 2000 AU from its star. Consequently, one year on GU Piscium b lasts 163 years. By comparison, Voyager 1 is approximately 160 AU from Earth!
However, GU Piscium b isn't an ice-cold planet like Uranus and Neptune; it boasts temperatures double that of Venus.
COCONUTS-2b takes the concept of distance to an extreme, orbiting its red dwarf star at a staggering 7500 AU, resulting in a year that exceeds one million Earth years. One would need an exceptionally long lifespan to celebrate a birthday in such a system.
The discussion of distant planets could continue endlessly. For example, the HR 8799 system contains four known planets, the closest of which takes 45 years to orbit, while the furthest requires 460 years.
Although fascinating, none of these planets qualify as true ice giants!
While our practical knowledge of ice giants remains limited, past microlensing studies have revealed that Neptune-mass planets are the most prevalent type of planet to form at considerable distances from their stars. A 2016 study highlighted that cold Neptune-like planets are significantly more likely to form than Jupiter-like planets in similar orbits.
This leads us to two important conclusions: we are fortunate to have Jupiter in our Solar System, and it is crucial to devote more attention to Uranus and Neptune. Further study of these ice giants will enhance our understanding of exoplanetary ice giants and, in turn, shed light on the origins of our Solar System.
Thank you for reading this article! If you would like to support a budding writer, please consider visiting my Ko-fi page!