WASP-193b is a hot, transiting

gas giant A gas giant is a giant planet composed mainly of hydrogen and helium. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" was originally synonymous with "giant planet". However, in the 1990s, it became known that Uranu ...

planet located approximately away in the constellation of Hydra, orbiting the

F-type star In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...

WASP-193. Its discovery was made by the WASP-South transit survey and announced in 2023. The planet is extremely bloated, with a radius nearly 50% larger than

Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a Jupiter mass, mass more than 2.5 times that of all the other planets in the Solar System combined a ...

, despite having only 14% of its mass. This places its density at , the second lowest of any known exoplanet as of May 2024 after Kepler-51d, and comparable to that of

cotton candy Cotton candy, also known as candy floss (candyfloss) and fairy floss, is a spun sugar confection that resembles cotton. It is made by heating and liquefying sugar, and spinning it centrifugally through minute holes, causing it to rapidly cool ...

(about ).

Discovery

The planet was discovered in July 2023 by a team of astronomers led by Khalid Barkaoui, a researcher at the

University of Liège The University of Liège (), or ULiège, is a major public university of the French Community of Belgium founded in 1817 and based in Liège, Wallonia, Belgium. Its official language is French (language), French. History The university was foun ...

, from observational data taken by WASP (Wide Angle Search for Planets) in 2006–2008 and 2011–2012. It is one of hundreds discovered in the WASP mission, which uses

transit photometry Methods of detecting exoplanets usually rely on indirect strategies – that is, they do not directly image the planet but deduce its existence from another signal. Any planet is an extremely faint light source compared to its parent star. For e ...

to find exoplanets, observing the dimming of a star caused by the

astronomical transit In astronomy, a transit (or astronomical transit) is the passage of a astronomical object, celestial body directly between a larger body and the observer. As viewed from a particular vantage point, the transiting body appears to move across the ...

of planets passing in front of them. The discovery was subsequently confirmed photometrically by TRAPPIST-South, SPECULOOS-South, and the

TESS Tess or TESS may refer to: Film and theatre * Tess (1979 film), ''Tess'' (1979 film), a 1979 film adaptation of ''Tess of the d'Urbervilles'' * Tess (2016 film), ''Tess'' (2016 film), a South African production Music * Tess (band), a Spanish pop ...

mission, as well as through

spectroscopic Spectroscopy is the field of study that measures and interprets electromagnetic spectra. In narrower contexts, spectroscopy is the precise study of color as generalized from visible light to all bands of the electromagnetic spectrum. Spectrosc ...

observations by

HARPS The High Accuracy Radial Velocity Planet Searcher (HARPS) is a high-precision Echelle grating, echelle planet-finding spectrograph installed in 2002 on the ESO 3.6 m Telescope, ESO's 3.6m telescope at La Silla Observatory in Chile. The First l ...

and the CORALIE spectrograph of the Swiss 1.2-metre Leonhard Euler Telescope. Due to WASP-193b's unusually low mass (and hence a weak

gravitational pull In physics, gravity (), also known as gravitation or a gravitational interaction, is a fundamental interaction, a mutual attraction between all massive particles. On Earth, gravity takes a slightly different meaning: the observed force be ...

) for a planet its size, initial observations failed to detect

radial velocity The radial velocity or line-of-sight velocity of a target with respect to an observer is the rate of change of the vector displacement between the two points. It is formulated as the vector projection of the target-observer relative velocity ...

signals in the spectra of WASP-193. Because of this, it took four years to gather sufficient data to determine the existence of a mass signal from the planet.

Host star

The planet orbits a yellow-white main-sequence star named WASP-193. The star has a mass of , a radius of , and a

luminosity Luminosity is an absolute measure of radiated electromagnetic radiation, electromagnetic energy per unit time, and is synonymous with the radiant power emitted by a light-emitting object. In astronomy, luminosity is the total amount of electroma ...

of . It has a surface temperature of and is billion years old. In comparison, the Sun is 4.6 billion years old, has a temperature of and a spectral type of G2V. The

apparent magnitude Apparent magnitude () is a measure of the Irradiance, brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction (astronomy), ...

of the star is 12.134, making it too faint to be seen from Earth by the naked eye, but visible using a 60 mm

aperture In optics, the aperture of an optical system (including a system consisting of a single lens) is the hole or opening that primarily limits light propagated through the system. More specifically, the entrance pupil as the front side image o ...

telescope A telescope is a device used to observe distant objects by their emission, Absorption (electromagnetic radiation), absorption, or Reflection (physics), reflection of electromagnetic radiation. Originally, it was an optical instrument using len ...

Physical characteristics

Orbit

The planet revolves around the star at a distance of just , over five times closer than Mercury is to the

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

(0.3871 AU). As a result, WASP-193b receives approximately 410 times more

irradiance In radiometry, irradiance is the radiant flux ''received'' by a ''surface'' per unit area. The SI unit of irradiance is the watt per square metre (symbol W⋅m−2 or W/m2). The CGS unit erg per square centimetre per second (erg⋅cm−2⋅s−1) ...

than the

solar constant The solar constant (''GSC'') measures the amount of energy received by a given area one astronomical unit away from the Sun. More specifically, it is a flux density measuring mean solar electromagnetic radiation ( total solar irradiance) per un ...

(i.e., the amount of energy received from the sun per given area at a distance of 1 AU), placing its

equilibrium temperature The planetary equilibrium temperature is a theoretical temperature that a planet would be if it were in radiative equilibrium, typically under the assumption that it radiates as a black body being heated only by its parent star. In this model, th ...

at a smoldering , hot enough to melt

silver Silver is a chemical element; it has Symbol (chemistry), symbol Ag () and atomic number 47. A soft, whitish-gray, lustrous transition metal, it exhibits the highest electrical conductivity, thermal conductivity, and reflectivity of any metal. ...

. Due to its exposure to intense stellar radiation, the upper layers of the planet's

atmosphere An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...

are being stripped away at a rate of 1.8 – 4.3 g per second, depending on the level of

extreme ultraviolet Extreme ultraviolet radiation (EUV or XUV) or high-energy ultraviolet radiation is electromagnetic radiation in the part of the electromagnetic spectrum spanning wavelengths shorter than the hydrogen Lyman-alpha line from 121 nm down to ...

flux Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications in physics. For transport phe ...

Size and density

WASP-193b has a radius of 1.464 (; 16.41 ), meaning the planet is approximately 3.1 times more voluminous than Jupiter. Despite its enormous size, the planet only has a mass of 0.139 (44.2 ; 2.58 times that of

Neptune Neptune is the eighth and farthest known planet from the Sun. It is the List of Solar System objects by size, fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 t ...

), making it a

super-Neptune A super-Neptune is a planet that is more massive than the planet Neptune. These planets are generally described as being around 5–7 times as large as Earth with estimated masses of 20–80 ; beyond this they are generally referred to as gas gian ...

, which normally has less than half the radius (5-7 ). Thus, the density of the planet is estimated to be a mere —well below the typical value seen in

s (), and less than a tenth that of

Saturn Saturn is the sixth planet from the Sun and the second largest in the Solar System, after Jupiter. It is a gas giant, with an average radius of about 9 times that of Earth. It has an eighth the average density of Earth, but is over 95 tim ...

(), the least dense planet in the

Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

. As of May 2024, only one other planet, the

super-puff A super-puff is a type of exoplanet with a mass only a few times larger than Earth's but with a radius larger than that of Neptune, giving it a very low mean density.

planet Kepler-51d (), is known to have a lower density. Because of this, both planets are often likened to cotton candy (around ) in media coverage.

Composition

Most of the planet's radius is thought to be taken up by a bloated atmosphere consisting of predominantly

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

and

helium Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...

, but it remains unknown as to how such a large, light planet could exist, which classical models for gas giant evolution fail to explain. Assuming the planet's age to be 4.4 Gyr, a

theoretical model A theory is a systematic and rational form of abstract thinking about a phenomenon, or the conclusions derived from such thinking. It involves contemplative and logical reasoning, often supported by processes such as observation, experimentation, ...

published in 2007 regarding the radii of planets aged between 1.0 and 4.5 Gyr with a

core Core or cores may refer to: Science and technology * Core (anatomy), everything except the appendages * Core (laboratory), a highly specialized shared research resource * Core (manufacturing), used in casting and molding * Core (optical fiber ...

mass of 0–10 calculates its radius to be 0.9–1.1 . Another model, published in 2013 and derived from the analysis of 35 exoplanets weighing less than 150 , gives a value of . Calculations using a 2018 model based on data from 286

hot Jupiters Hot Jupiters (sometimes called hot Saturns) are a class of gas giant exoplanets that are inferred to be physically similar to Jupiter (i.e. Jupiter analogues) but that have very short orbital periods (). The close proximity to their stars and hi ...

with known masses and radii predicts the radius to be . All of these models fall well behind the measured value of 1.464 , which, according to model calculations, can only be sustained for a few tens of millions of years, far shorter than the age of the host star, hinting at the presence of other previously unaccounted mechanisms at play. Research into the anomalously light WASP-193b is said to become crucial for understanding the evolution of inflated planets that cannot be explained by ordinary theories for planetary evolution. Due to the large transit depth, extremely low density, and high equilibrium temperature of the planet, it is considered a prime target for transmission photometry observations by the

James Webb Space Telescope The James Webb Space Telescope (JWST) is a space telescope designed to conduct infrared astronomy. As the largest telescope in space, it is equipped with high-resolution and high-sensitivity instruments, allowing it to view objects too old, Lis ...

Footnotes

References

{{2023 in space Exoplanets discovered by WASP Giant planets Hot Neptunes Hot Jupiters Hydra (constellation) Exoplanets discovered in 2023 Transiting exoplanets