The speed champion in our solar system is Mercury. Its average speed is 47 km/s. It is so fast that it is named after Mercury, the wing-footed God.
Jupiter was closest to the Sun. What if Jupiter was even hotter than Mercury?
Mercury is a distant neighbour of the Sun in a solar system that is 855 light-years away. This planet is one of the shortest ever measured, as it only takes 16 hours to circle its star. It is one of the most exotic planets ever found because of its distance and speed.
Astronomers call the planet TOI-2109b an Ultrahot Jupiter. Hot Jupiters are gas giants that are close to their stars and have high surface temperatures. Jupiters are even hotter. Their surface temperatures are more than 2200. The dayside temperature of TOI-2109 b is hotter than some small stars.
The paper presented the discovery. The paper is titled "TOI-2109: An Ultrahot Gas Giant on a 16 hr Orbit." Ian Wong is the lead author, but he is also a post-doc at MIT.
The planet was found in May 2020. TESS was watching it for almost a month. Multiple ground-based observatories made follow-up observations over the course of a year. The observations confirmed that TOI-2109b is a rare and unusual planet.
The study co-author said that everything was consistent with it being a planet.
The shortest period for a gas giant was measured by TOI-2109 b. The previous record-holder has an 18-hour time period. The planet is five times bigger than Jupiter and 1.5 times bigger than the Sun. It is hard to imagine what this arrangement would look like to any observer.
An artist's impression of a transiting Jupiter-mass exoplanet around a star that is slightly more massive than the Sun. The image is from NASA.
The planet is so hot because it is an average of only 2 million km from its star. It is locked to its star like other Hot Jupiters and Ultrahot Jupiters. The high dayside temperature can tear the molecule apart. Modelling shows that this can happen. The hydrogen can combine again if the night side is cooler.
The team was able to observe the planet as it traveled around its star. They watched the eclipse in multiple wavelength. The daytime temperature is likely to exceed 3500 K because TESS isn't sensitive enough. If it is true that hydrogen is torn apart on the dayside and recombines on the nightside, that could mean that the temperature isn't as extreme.
Shporer said that the planet's night side brightness is below the sensitivity of the TESS data, which raises questions about what is really happening there. Is the temperature very cold or does the planet take heat on the day side and transfer it to the night side? We are at the beginning of trying to answer this question.
The researchers found that TOI-2109b is slowly going into the star. Other Hot Jupiters have been found, but nothing like this.
A hot Jupiter is the concept of a Jupiter-sized exoplanet that is relatively close to its star. Credit: NASA/JPL-Caltech
Ultrahot and Hot Jupiters are one of the most extreme types of exoplanets. The team hopes that the Hubble will be able to study the planet, along with the soon-to-be-launched James Webb Space Telescope. As the planet gets closer to the star, it's interesting to watch.
Wong says that TOI-2109b is the most extreme subclass of exoplanet. Some of the unique physical and chemical processes that occur in their atmospheres have no parallels in our own solar system.
Future observations of TOI-2109b may reveal clues to how the systems come to be. Hot Jupiters have been seen as odd from the beginning of exoplanetary science. How does a planet of Jupiter's size get to an elliptical path that is only a few days long? We don't have anything like this in our Solar System, and we see this as an opportunity to study them and help explain their existence.
Jupiter may have migrated to within 1.5AU of the Sun in the distant past, and then reversed course to its current path. That is called the Grand Tack hypothesis. It would have been something to behold.
Jupiter moved toward the Sun. Their fates became linked as they did. The pair moved away from the Sun when Jupiter was near Mars. Scientists refer to this as the Grand Tack, a reference to the sailing maneuver. The credit is NASA/GSFC.
We learn a lot about the range of planet types out there when we find extreme and unusual exoplanets. There are lots of Hot Jupiters and Ultrahot Jupiters because they are close to their stars. They are actually very rare.
The authors say that only a small percentage of Sun-like stars host these planets. Even though their numbers are small, they make a huge contribution to our understanding of exoplanets. Their large size in relation to their host stars and high temperatures allow a broad range of intensive studies that extend far beyond the rudimentary measurements of planet mass and radius.
In their paper, they write that over the past two decades, a wide arsenal of observational techniques has been used to probe the atmospheric properties of hot Jupiters. It is becoming easier to study things like temperature distribution, chemical composition, and photochemical hazes.
Ultrahot Jupiters are characterized by a number of distinct physical and dynamical properties that set them apart from the rest of the hot gas-giant population.
No article on exoplanets can be complete without looking at the James Webb Space Telescope. The power of the JWST will allow it to probe exoplanet atmospheres more rigorously than any other tool currently available.
Finding Earth-like planets in the right places is a part of the search for exoplanets. Ultrahot Jupiters like TOI-2109b can teach us a lot about planets at their most extreme and about planet-star interactions that we can't study in our Solar System. The JWST will make a significant contribution to our knowledge.
Ultrahot Jupiters will continue to be one of the most fruitful candidates for impactful efforts at characterization, providing crucial insights into the nature of planets at their most extreme, despite future advances in telescope capabilities.