Astronomers have recently unveiled a fascinating discovery: a giant planet, WASP-193b, with a density so low it rivals that of cotton candy. Located 1,232 light-years away in the Milky Way galaxy, this massive yet airy planet presents a perplexing challenge to existing planetary formation theories. WASP-193b, discovered through the Wide Angle Search for Planets (WASP) initiative, is about 50% larger than Jupiter but only a tenth of its density, making it the second-lightest planet ever discovered.
The planet’s existence challenges conventional wisdom about gas giants. Its remarkably low density, comparable to 0.059 grams per cubic centimeter, makes it an extreme outlier amongst the over 5,400 planets discovered so far. “To find these giant objects with such a small density is really, really rare,” says lead study author Khalid Barkaoui, a postdoc at MIT. “There’s a class of planets called puffy Jupiters, and it’s been a mystery for 15 years now as to what they are. And this is an extreme case of that class.”
WASP-193b’s discovery was initially prompted by observations of its host star, WASP-193, a sun-like star. Astronomers noticed periodic dips in the star’s brightness, suggesting a planet was transiting, or passing in front of it, every 6.25 days. The amount of light blocked during these transits revealed the planet’s impressive size, hinting at its unusual nature.
The next step was to determine the planet’s mass, a crucial factor in calculating its density and understanding its composition. Typically, astronomers employ the radial velocity technique to measure a planet’s mass. This method analyzes shifts in a star’s spectrum caused by the gravitational pull of an orbiting planet. However, WASP-193b proved too light to induce any noticeable shifts in its host star’s spectrum, making traditional mass estimation impossible.
Typically, big planets are pretty easy to detect because they are usually massive, and lead to a big pull on their star,
explains Julien de Wit, an assistant professor at MIT’s Department of Earth, Atmospheric and Planetary Sciences. “But what was tricky about this planet was, even though it’s big — huge — its mass and density are so low that it was actually very difficult to detect with just the radial velocity technique. It was an interesting twist.”
Despite the challenges, the team successfully confirmed the planet’s extraordinarily low mass, approximately 0.14 times that of Jupiter. “We were initially getting extremely low densities, which were very difficult to believe in the beginning,” adds co-lead author Francisco Pozuelos, a senior researcher at the Institute of Astrophysics of Andalucia in Spain. “We repeated the process of all the data analysis several times to make sure this was the real density of the planet because this was super rare.”
Scientists speculate that WASP-193b primarily consists of hydrogen and helium, similar to most gas giants. However, its extremely low density suggests an incredibly inflated atmosphere, extending far beyond Jupiter’s. This raises a fundamental question: how can a planet inflate to such an extent while remaining so light? Current planetary formation models struggle to explain this phenomenon.
To shed more light on this enigmatic world, astronomers plan to utilize a technique developed by de Wit to analyze WASP-193b’s atmosphere. This will reveal properties like temperature, composition, and pressure at different depths, providing valuable insights into the planet’s mass and formation history.
WASP-193b stands as an ideal candidate for further study by powerful observatories like the James Webb Space Telescope. “The bigger a planet’s atmosphere, the more light can go through,” says de Wit. “So it’s clear that this planet is one of the best targets we have for studying atmospheric effects. It will be a Rosetta Stone to try and resolve the mystery of puffy Jupiters.”
The original research article can be accessed here.
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