Astronomers may have inadvertently complicated the mystery of how strange “roasting marshmallow” planets form. Using the Gemini South telescope, researchers found that the “hot and puffy” ultra-hot Jupiter planet WASP-121b may have formed closer to its star than previously believed, challenging what we know about how planets form.
Since the discovery of the first planet outside the solar system in the mid-1990s, the catalog of extrasolar planets, or “exoplanets,” has grown to over 5,000 entries. Many of these exoplanets are like nothing found in our solar system. The hot and ultra-hot Jupiters are prime examples of this, being gas giant planets many times the mass and size of Jupiter that are so close to their stars that they can complete an orbit in a matter of a few hours.
Up to one-third of the exoplanets discovered so far are hot Jupiters. These hot, puffy worlds endure extreme temperatures, making them very aptly nicknamed “roasting marshmallows.” These planets are thought to form farther away from their stars in orbits similar to those of Jupiter and Saturn in our own solar system, before migrating inwards. However, the new study of WASP-121b throws those origin ideas into doubt.
The team behind the new research came to this conclusion when they began studying the chemistry of protoplanetary disks, the flattened clouds of gas and dust around infant stars from which planets emerge, using the Immersion GRating INfrared Spectrograph (IGRINS) instrument on the Gemini South telescope in Chile.
With IGRINS the team was able to measure the ratio the rock-to-ice ratio for a transiting planet using a single instrument for the first time ever. Their measurement eliminated possible errors that could arise for other instruments, proving a powerful new way to perform the chemical analysis of exoplanets.
“Ground-based data from Gemini South using IGRINS actually made more precise measurements of the individual chemical abundances than even space-based telescopes could have achieved,” Peter Smith of the Roasting Marshmallows Program said in a statement.
“Our instrument sensitivity is advancing to the point where we can use these elements to probe different regions, altitudes, and longitudes to see subtleties like wind speeds, revealing just how dynamic this planet is.”
Did WASP-121b form next to its star?
Located around 858 light-years from Earth, WASP-121b has 1.2 times the mass of Jupiter but is puffed out, making it 1.9 times the width of the solar system’s largest planet. It is so close to its star that it takes just 1.3 Earth days to complete an orbit. WASP-121b is tidally locked, meaning the planet has a scorching hot “dayside” that permanently faces that stellar parent and a cooler night side that perpetually faces out into space.
The dayside of WASP-121b is so hot at around 4,500 degrees Fahrenheit (2,500 degrees Celsius) that metals on the planet can be vaporized and drift upward into its atmosphere. These metals are then blown to the planet’s nightside by powerful 11,000-mph (17,700 km/h) winds, where they cool and fall as rains of liquid metal, ruby, and sapphire.
Standard planetary formation models suggest that WASP-121b should have formed further out in the protoplanetary disk that once surrounded its star than the position it occupies today before then migrating inwards. But the planet’s chemistry doesn’t seem to back this idea.
A gradient should have existed in the protoplanetary disk of this system (and all others) that saw rocky and icy materials change from vapor to solid as the distance from the star increased.
Astronomers can hunt for signatures of elements in planets and their atmospheres and determine the ratio of rocky matter to icy gaseous matter present when the planet was born. That should tell them how far away from the star the planet formed.
To determine this ratio, astronomers usually have to make repeated observations with different instruments: a visible light instrument to detect solid rocky material and an infrared instrument to detect gaseous matter.
The fact that WASP-121b is so hot means both these types of elements are vaporized in its atmosphere and can be detected with IGRINS as the planet crosses or “transits” the face of its star.
“The climate of this planet is extreme and nothing like that of Earth,” Smith said. “The planet’s dayside is so hot that elements typically thought of as ‘metal’ are vaporized into the atmosphere, making them detectable via spectroscopy.”
Thus, with IGRINS, this team was able to discover the rock-to-ice ratio of WASP-121b, which was particularly high. This suggests that in the planet’s infancy it was able to accrete a great deal of rocky matter as it was forming. That would indicate that it was born in a region of the protoplanetary disk that was too hot for ices to condense. This was a surprise for scientists as the current paradigm suggests gas giants need solid ices to form.
“Our measurement means that perhaps this typical view needs to be reconsidered and our planet formation models revisited,” Smith added.
Smith and colleagues now intend to expand their investigation of ultra-hot Jupiters in other planetary systems using the upgraded IGRINS-2 instruments currently being calibrated and ready for use.
This should allow scientists to build a larger sample of hot Jupiter exoplanet atmospheres and unlock the secrets of these extreme worlds unlike anything seen in the solar system.
The team’s research was published on Dec. 2 in The Astronomical Journal.
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