A remote hot Jupiter exoplanet has foregone the typical solitary life that worlds of its kind normally lead, in favor of companionship with another planet — and now, astronomers think they know why.
Hot Jupiters are gas giants that orbit exceedingly close to their star. However, they don't form that close, but are rather assembled much farther out before migrating inward. When they make that trip, they usually kick out any other planets in their way — but the hot Jupiter TOI-1130c seems to have latched onto a smaller planet as a traveling companion. The two seem to have migrated towards their star together.
"This was a one-of-a-kind system," said Huang in a statement. "Hot Jupiters are 'lonely', meaning that they don't have companion planets inside their orbits. They are so massive and their gravity so strong, that whatever is inside their orbit just gets scattered away. But somehow, with this hot Jupiter, an inner companion has survived, and that raises questions about how such a system could form."
Now an international team of astronomers led by MIT's Saugata Barat and including Huang think they have found the answer by bringing the James Webb Space Telescope (JWST) to bear on the hot Jupiter's companion, which is a mini-Neptune type world three-and-a-half times the diameter of Earth and catalogued as TOI-1130b. By observing the system when the mini-Neptune was transiting its star, they were able to search for where the planet's atmosphere was absorbing the star's light. The wavelengths of light being absorbed told them that the planet sports a "heavy" atmosphere full of water vapor, carbon dioxide, sulfur dioxide and signs of methane. In this context, "heavy" means heavier than hydrogen and helium, elements which would be expected to dominate the atmosphere if the mini-Neptune had formed close to its star.
Instead, the mini-Neptune and the hot Jupiter must have formed beyond the snow line — sometimes called the "frost line" — which is the distance in the protoplanetary disk that bequeathed the planets where temperatures were cold enough for water to be ice rather than liquid or vapor.
"This is the first time we've observed the atmosphere of a planet that is inside the orbit of a hot Jupiter," said Barat. "This measurement tells us this mini-Neptune indeed formed beyond the 'frost line'."
The mini-Neptune survived rather than get bundled out of the way by the marauding hot Jupiter because the two metaphorically held hands while migrating in together, before finally settling into their current orbits where they are anchored by a gravitational resonance between them.
In their current orbits, TOI-1130b orbits its star every four days at a distance of 4.2 million miles (6.8 million kilometers or 0.0453 astronomical units, AU) and with a temperature of 1,025 degrees Fahrenheit (550 degrees Celsius). Meanwhile, TOI-1130c orbits every eight days at a distance of 6.8 million miles (10.9 million km or 0.0731 AU), which is close enough to reach a temperature of 930 degrees F (500 degrees C). In other words, the planets are in a 2:1 resonance in which the mini-Neptune orbits twice for every one orbit of the hot Jupiter.
However, the gravitational ties between the two planets also posed a challenge for Barat's team to observe them with the JWST.
Because the two worlds tug on each other gravitationally, pulling each other around or holding each other back at different points in their orbits, it leads to transit timing variations, or TTVs — discrepancies in when they are expected to transit their star. With time on the JWST highly sought after, Barat's team would only get one chance to observe the planets and if they miscalculated and observed at the wrong time, they would miss them.
"It was a challenging prediction and we had to be spot on," said Barat.
To accomplish this, Judith Korth of Lund University in Sweden developed a model based on past observations of the system with which to predict when each planet would be transiting. The model worked a treat, with JWST's observations not only explaining the TOI-1130 system, but possibly all mini-Neptunes that are found close to their star.
"This system represents one of the rarest architectures that astronomers have ever found," summarized Barat. "The observations of TOI-1130b provide the first hint that such mini-Neptunes that form beyond the water/ice lines are indeed present in nature."
The findings were published on May 5 in The Astrophysical Journal Letters.
