WASP-193b
An extremely low-density gas giant, second only to Kepler-51d.
WASP-193b is a gas giant exoplanet that orbits the F-type star WASP-193, located about 1232 light-years away in the constellation Hydra. It transits its host star and is intensely hot. The planet is remarkably puffy: its radius is nearly 50% larger than Jupiter’s, yet it contains only 14% of Jupiter’s mass. As of May 2024, this gives it the second-lowest density of any known exoplanet, after Kepler-51d, and a density comparable to cotton candy.
The planet was discovered in July 2023 by a team led by Khalid Barkaoui of the University of Liège. They used data from the WASP-South transit survey, collected between 2006–2008 and 2011–2012. WASP detects exoplanets by measuring the dimming of a star when a planet passes in front of it. The discovery was confirmed photometrically by TRAPPIST-South, SPECULOOS-South, and TESS, and spectroscopically by HARPS and the CORALIE spectrograph on the Swiss 1.2-metre Leonhard Euler Telescope.
Because WASP-193b is so light for its size, its gravitational pull is weak. Initial observations could not detect radial velocity signals in the star’s spectrum. It took four years of data to confirm a mass signal from the planet.
The host star, WASP-193, is a yellow-white main-sequence star. It has a surface temperature of 6078 K and is about 4.4 billion years old—slightly younger than the Sun, which is 4.6 billion years old and has a temperature of 5772 K. The star’s apparent magnitude is 12.134, too faint to see with the naked eye but visible through a 60 mm aperture telescope.
WASP-193b orbits just 0.0676 AU from its star—over five times closer than Mercury is to the Sun. It receives about 410 times the solar constant’s energy, giving it an equilibrium temperature of 1254 K, hot enough to melt silver. Intense stellar radiation strips its upper atmosphere at a rate of 1.8 to 4.3 grams per second, depending on extreme ultraviolet flux.
The planet’s radius is 1.464 times Jupiter’s, making it about 3.1 times more voluminous. Its mass is only 0.139 times Jupiter’s (44.2 Earth masses, or 2.58 times Neptune’s), classifying it as a super-Neptune—though super-Neptunes normally have radii less than half that. Its density is far below that of typical gas giants, less than a tenth of Saturn’s (the Solar System’s least dense planet). Only Kepler-51d is known to be less dense.
Quick Facts
- Discoverer
- Khalid Barkaoui et al.
- Discovered
- July 2023 (announced)
- Discovery Method
- Transit method
- Apsis
- astron
- Eccentricity
- 0.0560 · 0.0680 · 0.0400
- Star
- WASP-193
Facts from the source article.
Lore & Background
WASP-193b was discovered in July 2023 by a team led by Khalid Barkaoui of the University of Liège, using data from the WASP-South transit survey collected between 2006–2008 and 2011–2012. The discovery was confirmed photometrically by TRAPPIST-South, SPECULOOS-South, and TESS, and spectroscopically by HARPS and the CORALIE spectrograph on the Swiss 1.2-metre Leonhard Euler Telescope. Because of the planet's unusually low mass and weak gravitational pull, initial radial velocity observations failed to detect a signal, requiring four years of data to confirm the mass.
The planet orbits its yellow-white main-sequence star at just 0.0676 AU, over five times closer than Mercury is to the Sun, receiving about 410 times Earth's solar irradiance. Its equilibrium temperature is 1254 K, hot enough to melt silver. The upper atmosphere is being stripped at a rate of 1.8–4.3 grams per second depending on extreme ultraviolet flux. Despite its enormous size—3.1 times Jupiter's volume—WASP-193b has only 0.139 Jupiter masses, making it a super-Neptune with a density well below that of Saturn, the least dense planet in the Solar System.
Most of the planet's radius is thought to be a bloated atmosphere of hydrogen and helium. Classical gas giant evolution models fail to explain its existence: theoretical models from 2007, 2013, and 2018 predict radii of 0.9–1.1, 1.0–1.2, and 1.0–1.2 Jupiter radii respectively, all far below the measured 1.464. Model calculations suggest such a radius can only be sustained for tens of millions of years, much shorter than the host star's age, hinting at unknown mechanisms.
Reader's Guide
WASP-193b is significant as one of the most extreme examples of an inflated exoplanet, challenging standard models of planetary formation and evolution. Its anomalously low density—second only to Kepler-51d as of May 2024—places it in a category of 'super-puff' planets that cannot be explained by ordinary theories. The planet's large transit depth, extremely low density, and high equilibrium temperature make it a prime target for transmission photometry observations by the James Webb Space Telescope. Research into WASP-193b is considered crucial for understanding how such inflated planets can exist and persist, as its measured radius is far larger than what models predict for a planet of its age and mass. The discovery also highlights the difficulty of detecting low-mass planets via radial velocity, as the weak gravitational signal required years of data to confirm. Its comparison to cotton candy in media coverage underscores its extraordinary nature, and it joins a small group of similarly puffy worlds like Kepler-51 b, c, d, WASP-17b, and HAT-P-67b that push the boundaries of planetary science.
Did You Know?
- WASP-193b has a density comparable to cotton candy, the second lowest of any known exoplanet as of May 2024.
- The planet's radius is 1.464 times Jupiter's, but its mass is only 0.139 Jupiter masses.
- It took four years of radial velocity data to confirm the planet's mass due to its weak gravitational pull.
- The planet's equilibrium temperature of 1254 K is hot enough to melt silver.
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