The James Webb Space Telescope (JWST) has provided groundbreaking insights into the unusual exoplanet WASP-107 b, uncovering the mechanisms behind its unexpectedly low density.
This discovery marks a significant step in understanding exoplanet formation and evolution, challenging previous theories and offering new explanations for the planet's peculiar characteristics.
The Mystery of WASP-107 b
WASP-107 b has intrigued astronomers since its discovery in 2017 by the Wide Angle Search for Planets (WASP) consortium. Located about 210 light-years from Earth in the constellation Virgo, this "warm Neptune" exoplanet orbits its star at a distance of only 5 million miles, completing an orbit every 5.7 days. Despite its proximity to its star, WASP-107 b maintains a moderate temperature and an ultra-low density, comparable to a microwaved marshmallow.
The planet's density defies standard theories of planet formation. Although it is 80% the size of Jupiter in terms of volume, it has less than 10% of Jupiter's mass, making it one of the least dense exoplanets known. This low density posed a significant puzzle: How could a planet with such a small rocky core accumulate a massive gaseous envelope and remain so inflated without an external heat source?
Breakthrough Observations
Two independent research teams have now shed light on this mystery using data from the James Webb Space Telescope (JWST), combined with previous observations from the Hubble Space Telescope. The key finding is the surprisingly low amount of methane in WASP-107 b's atmosphere, which is about one-thousandth of what would be expected for a planet of its temperature.
Methane is typically unstable at high temperatures, and its deficiency suggests that WASP-107 b's interior is much hotter than previously estimated. This conclusion points to tidal heating as the primary mechanism responsible for the planet's puffiness. Tidal forces arise from the planet's slightly elliptical orbit, which causes continuous gravitational stretching and heating as the distance between the planet and its star varies. This process is similar to how the Moon's gravitational pull causes tides on Earth.
Detailed Atmospheric Analysis Done by the James Webb Space Telescope
The JWST's advanced instruments enabled researchers to perform a detailed analysis of WASP-107 b's atmosphere. By using transmission spectroscopy, they could measure the amounts of different gases present. This technique involves observing the planet-star system before, during, and after the planet transits across the face of the star. By comparing the brightness of light filtered through the planet's atmosphere to unfiltered starlight, scientists can determine the abundance of various gases, as each molecule absorbs a unique combination of wavelengths.
The transmission spectrum obtained from JWST and Hubble observations showed clear evidence for water vapor (H2O), carbon dioxide (CO2), carbon monoxide (CO), sulfur dioxide (SO2), and ammonia (NH3) in WASP-107 b's atmosphere. This broad spectrum coverage, ranging from optical to mid-infrared wavelengths, is the most comprehensive for any exoplanet to date and includes the first space telescope detection of ammonia in an exoplanet atmosphere.
Implications of Tidal Heating
The findings on WASP-107 b suggest that tidal heating plays a crucial role in the planet's inflated atmosphere. The continuous stretching and compressing of the planet due to its elliptical orbit generate significant internal heat, which prevents the atmosphere from contracting. This internal heating also affects the atmospheric chemistry, reducing methane levels and increasing the presence of other carbon-bearing molecules.
The realization that tidal heating can inflate an exoplanet's atmosphere without the need for extreme formation theories has broader implications. It provides a new framework for understanding other low-density exoplanets and offers a potential explanation for the puffiness observed in dozens of similar planets.
Core Mass and Formation
Another significant outcome of the research is the revised estimation of WASP-107 b's core mass. The internal heating and subsequent mixing of atmospheric gases suggest that the planet's core is more massive than originally thought.
This finding aligns with the notion that planets like WASP-107 b did not form in an unusual manner with a super small core and a huge gaseous envelope. Instead, they may have formed more like Neptune, with a substantial rocky core and a moderate amount of gas, which later expanded due to tidal heating.
Future Research and Exploration
The insights gained from WASP-107 b demonstrate the power of the James Webb Space Telescope in studying exoplanet atmospheres and internal dynamics. The ability to detect and measure a wide range of molecules provides invaluable information about the composition, temperature, and evolutionary history of these distant worlds.
As JWST continues its mission, it will undoubtedly uncover more secrets about exoplanets and their formation processes. The detailed observations of WASP-107 b are just the beginning, and future studies will build on this knowledge to explore other intriguing exoplanets with unusual characteristics.