A strange planet orbiting a distant star may be even weirder than we realized. Already thought to have “iron rain” and an unusual polar orbit, this ultrahot Jupiter might also have begun life far away from its star before diving into a tight 30-hour orbit.
The planet, WASP-121b, or Tylos, is about 850 light-years from Earth and was discovered in 2015. Observing the planet in October 2022 with the James Webb Space Telescope (JWST), researchers found that it hosted a surprising amount of methane and silicon monoxide. Their observations mark the first time silicon monoxide has been conclusively found on another world.
“Something weird happened dynamically in its past.”
The presence of methane and silicon monoxide, researchers say, might mean WASP-121b initially formed much farther from its star—as far away as 30 astronomical units, about the same distance Neptune lies from our Sun. (One astronomical unit is the average distance between the Sun and Earth.) The findings were published in Nature Astronomy and The Astronomical Journal.
“Something weird happened dynamically in its past,” said Tom Evans-Soma, an astronomer at the University of Newcastle in Australia and lead author of the Nature paper. “And it may be a big factor in how it moved from far out to close in.”
Iron Rain
Hot Jupiters are a class of gas giant planets that orbit extremely close to their stars and have temperatures exceeding 1,500 K (2,200°F). Ultrahot Jupiters are even closer and hotter, sometimes reaching temperatures above 2,000 K (3,100°F).
WASP-121b is one such ultrahot world, orbiting its star (WASP 121) within 2 times the star’s radius. At this proximity, the planet is tidally locked to the star, the way the Moon is to Earth, so the same face always points to the star. Atmospheric temperatures on WASP-121b can reach more than 3,000 K (4,900°F) on the dayside and 1,100 K (1,500°F) on the nightside.
This discrepancy in temperature may help explain the concept of iron rain on WASP-121b. Metals are likely to vaporize on the fiery dayside, and as these particles blow to the nightside, the drop in temperature creates conditions for droplets of liquid metal to form and fall from the planet’s atmosphere. “The nightside temperatures drop low enough for a whole bunch of these materials to condense,” possibly within seconds, said Evans-Soma.
The planet’s proximity to its star has also stretched the world into an oblong shape, and it orbits its star in a strange 90° orientation, almost pole to pole above and below the star. The planets of our solar system, by comparison, orbit in a flat plane.
A Distant Origin
These characteristics alone had already painted WASP-121b as an unusual world, but the latest observations further add to its mystery.
The researchers used JWST to observe the planet for 40 hours and pick apart its light, revealing the presence of water, carbon monoxide, and silicon monoxide on the dayside. These compounds may have been pulled from the nightside by a powerful equatorial jet with wind speeds of up to 10 kilometers (6 miles) per second.
The team detected methane in the planet’s nightside—a surprising result because methane shouldn’t survive WASP-121b’s high temperatures.
The team also detected methane in the planet’s nightside—a surprising result because methane shouldn’t survive WASP-121b’s high temperatures at all. “People have been looking for methane in exoplanets, but generally focusing on much cooler planets,” said Evans-Soma.
The presence of methane suggests the planet has a source of the compound replenishing its atmospheric supply. The team thinks the source might be trapped methane pulled up from the planet’s interior by strong convection currents.
The presence of methane might also point to WASP-121b forming much farther from its star. At a greater distance, icy pebbles of the methane were more abundant. Here, too, the gas giant may have consumed 21 Earths’ worth of rocky material during its formation, which would explain the presence of silicon.
A Starward Migration
Richard Booth, a planet formation expert at the University of Leeds in the United Kingdom who was not involved in the research, said that in general, scientists think hot Jupiters migrate inward over time. It is unlikely the planets formed close to their stars, he explained, because the stars’ gravity would have been too strong for planets to coalesce.
“Hot Jupiters definitely don’t form in situ,” said Booth.
But finding “evidence for migration is hard,” he continued, because migration can happen quickly (at least on planetary timescales)—in just millions or even thousands of years.
The WASP-121 system is thought to have formed about 1.1 billion years ago, with its migration possibly happening as a result of a gravitational nudge from a passing star or other planets in the system. Such a nudge might also explain the planet’s odd orbit.
Future work could tell us how this seemingly strange exoplanet compares with other ultrahot Jupiters. “It’s not clear that it is particularly unusual,” said Evans-Soma. “It just happens to be one of the planets we can study in really exquisite detail.”
—Jonathan O’Callaghan, Science Writer