What is WASP-107b and why it matters
WASP-107b is a warm sub-Neptune exoplanet about 200 light-years away in the constellation Virgo, discovered in 2017. It is known for an extremely low density that has been compared to cotton candy, because its mass is low relative to its size. This combination makes it one of the least dense planets found around a main-sequence star. Its bloated radius and weak gravity holding onto a modest atmosphere help planetary scientists test models of how gas-rich planets form and evolve. Below are the key properties, discovery context, and what WASP-107b teaches us about planet populations.
| Attribute | Verified Detail | Source Type |
|---|---|---|
| Planet name | WASP-107b | Designation |
| Host star | WASP-107, an M-type dwarf | Catalog |
| Constellation | Virgo | Coordinates |
| Discovery year | 2017 | Publication (SPECULOOS / WASP) |
| Distance | Approximately 200 light-years | Parallax-based estimate |
| Planet type | Warm sub-Neptune / gas-rich | Classification |
| Density | Very low; among the least dense known exoplanets | Radius and mass from transit and radial velocity |
| Orbital period | About 5.7 terrestrial days | Timing of transits |
| Equilibrium temperature | Approximately 600 K | Modeling of stellar irradiation |
| Notability | Used to study inflation and atmospheric escape | Scientific usage |
Discovery and context
WASP-107b was identified by the SuperWASP and SPECULOOS projects, which monitor bright stars for periodic dips caused by transiting planets. Follow-up radial velocity measurements from ground-based spectrographs refined its mass and radius. The planet orbits a relatively cool M-dwarf, which makes transit timing and atmospheric studies practical. Its position among known exoplanets is notable: it sits in a regime where planets are frequently described as gas-rich and puffed up, helping researchers distinguish between formation channels and mechanisms that remove or retain envelopes.
Key identification details
- Detection method: Transit photometry and radial velocity
- Primary survey: WASP and SPECULOOS
- Host-star type: M dwarf
- Subsequent studies: Hubble and ground-based spectroscopy to probe atmosphere
Physical characteristics and structure
WASP-107b has a radius similar to that of Neptune but only a small fraction of Neptune’s mass. This low mass-to-radius ratio produces a density far below that of water, inspiring the cotton candy analogy. The modest gravitational pull means lighter atmospheric components can be studied without the planet holding onto a deep, heavy envelope. Its classification as a warm sub-Neptune reflects both its size and its equilibrium temperature, which is hot compared to solar system planets but moderate by exoplanet standards.
Structure at a glance
| Property | Approximate Value | How it is known |
|---|---|---|
| Radius | Roughly 0.4 Jupiter radii | Transit light curves |
| Mass | Only a few Earth masses | Radial velocity |
| Mean density | Very low (among lowest known) | Combined transit + RV |
| Orbital distance | Close-in, well within the star’s corona | Timing of transits |
| Equilibrium temperature | Approximately 600 K | Stellar models + observations |
Atmosphere and evolution
Because WASP-107b has lower surface gravity than a typical Neptune, its atmosphere is easier to probe and less strongly bound. Studies using space telescopes have looked for signatures of gases and clouds, helping to constrain how much hydrogen and helium the planet retains. The planet appears to be experiencing atmospheric escape, with lighter elements escaping more readily than heavier ones. This process is important for understanding how planets evolve over time, how they lose their envelopes, and when they transition from gas-rich to rocky remnants. The host star’s activity also plays a role in atmospheric chemistry and long-term mass loss rates.
What atmospheric research has revealed
- No extremely deep, opaque cloud deck detected in some wavelengths
- Signatures of molecules consistent with moderate-temperature chemistry
- Evidence of ongoing atmospheric escape, especially of lighter species
Why WASP-107b is useful for planet science
WASP-107b serves as a benchmark for theories of planet inflation and atmospheric evolution. Its combination of low density, a well-constrained orbit, and an accessible atmosphere makes it a target for comparative studies. By comparing it to other warm sub-Neptunes and slightly more massive planets, researchers can test how migration, formation timing, and stellar environment shape final properties. The planet also demonstrates how projects like WASP and SPECULOOS complement each other: wide-field surveys find candidates, while targeted programs follow up the most scientifically valuable systems.
Comparisons with similar planets
| Planet | Density (relative) | Key distinction |
|---|---|---|
| WASP-107b | Among the lowest for warm sub-Neptunes | Benchmark for puffiness and atmospheric studies |
| Gliese 3470 b | Also low density, but cooler and smaller | Different host-star type and irradiation level |
| Kepler-51d | Similar low density, slightly hotter | Younger system with different evolution stage |
Broader implications and future work
Studying WASP-107b helps refine how planet-forming disks produce gas-rich cores, when rapid gas capture occurs, and how close-in planets are sculpted by their stars. Upcoming observations with more sensitive spectrographs and space missions can further constrain atmospheric composition and the exact rate of mass loss. These data will improve population models and refine predictions for how common low-density planets are in the Galaxy. For now, WASP-107b remains a textbook example of a lightweight, puffed-up world that bridges the gap between Neptune-like planets and hotter gas-rich bodies.
Ongoing monitoring of its host star and continued atmospheric characterization will keep the planet relevant for years. Its modest size and the practicality of observing it from both space and ground make WASP-107b a long-lived reference point in the study of planet demographics and evolution.