NASA, ESA, CSA, Joseph Olmsted (STScI)
The discovery of the TOI-2076 system, which scientists are calling the “Teenage Solar System,” is providing more clues about photoevaporation, a short but crucial phase in its development.
The stable Solar System we see around us today took time to develop. Not only did planetary orbits need time to stabilize, but so did planetary atmospheres. needed time to evolve. In fact, planetary orbits and evolving atmospheres work together to determine the final appearance of a solar system, and photoevaporation drives this process.
A photoevaporation occurs when ultraviolet and/or X-ray radiation from a star heats and ionizes gas in a planet’s atmosphere or in a protoplanetary disk, dissipating and removing it. This removes mass, and the loss of mass affects the orbital arrangement of the planets.
Photoevaporation is a brief but crucial step in the development of a solar system, and this step occurred billions of years ago in our system. But how exactly does it work?
Observations of what researchers are calling teenage solar system may contain some answers. The system is called TOI-2076 and was first discovered by TESS in 2020. A new investigation in Nature Astronomy presents these observations. The article’s main author is Mu-Tian Wang, from the School of Astronomy and Space Sciences at Nanjing University, China.
“We present a complete characterization of the TOI-2076 system, whose age of approximately 200 million years makes it a fundamental landmark for the study of the dynamic evolution and erosion of primordial atmospheres”, write the authors.
They explain that young solar systems often exhibit mean motion resonances (RMM). These resonances occur when multiple planets have orbits that are simple integer ratios to each other. These RMMs are often interrupted, something predicted by the Nice model.
The Nice model basically states that the giant planets in our Solar System formed close to each other and migrated to their current positions through gravitational interactions. This led to Intense Late Bombinga period that may explain the enormous number of craters observed on the surfaces of rocky bodies in the Solar System. This occurs after photoevaporation, a phase in which the young star dissipates gas in its protoplanetary disk and also removes planetary atmospheres with its radiation. This changes the orbital relationships between the planets.
Photoevaporation is an important phase in the development of a Solar System. It is a phase of transformation between a young solar system and a more mature one like ours. Research shows that photoevaporation can mitigate the disruption of radiation mass ratios (MMR) in planetary systems, depleting gas between planets and stabilizing orbits. The photoevaporation period can also erode planetary atmospheres.
“The transformation period is very short compared to the entire lifetime of the system,” said Howard Chen, co-author from the Department of Aerospace, Physical and Space Sciences at the Florida Institute of Technology. “This period is really crucial to determine how the system will develop into its mature state.”
“Here we present a detailed characterization of the TOI-2076 system, around 200 million years old, which contains four sub-Neptune planets with diameters between 1.4 and 3.5 Earth radii,” the authors write. “We demonstrated that their planets are close to, but not trapped in, mean-motion resonances, making the system dynamically fragile.”
TOI-2076 is a young K-type star only about 210 million years old. Its four planets orbit in a nearly consistent sequence, evidence that they were once much closer to each other but are slowly migrating outward. Observations also show that all have rocky cores. But their atmospheres are different. The planet closest to the star lost its atmosphere due to photoevaporation, while the three outermost ones did not suffer the effects to such an extent.
Chen works with computer models of planetary evolution, and these models show that planets gradually lose their atmospheres due to photoevaporation. What remains unclear is how long it takes and exactly how photoevaporation affects a young solar system.
Chen used his models to simulate how photoevaporation would shape the evolution of planets from birth to adolescence. With observations from the TOI-2076 system in hand, the researchers compared them to their models.
They found agreement between the models and observations. Over time, the planets evolved to resemble the planets in the TOI-2076 system. This means that photoevaporation was playing an intense role in the formation of evolving planets. The star’s powerful radiation stripped the planets’ atmospheres over time, with those furthest from the star retain more atmospheres. Photoevaporation removes mass, and the planets gradually moved away from each other as their atmospheres were removed.
“This trend is consistent with the loss of atmospheric mass due to photoevaporation, which predicts the envelopes of irradiated planets to erode completely or stabilize at a residual level of approximately 1% of mass in the first million years, with the most distant and least irradiated planets retaining most of their primordial envelopes”, explain the researchers.
“For me, the main goal of modeling is to connect results with observations. We want our models to reflect the real world, but that doesn’t always happen,” Chen said in a press release. “Seeing the model work in the real world and explaining what is happening is very impactful.”
The results also show that most solar systems should stabilize after the photoevaporation phase. After about 100 million yearsthe system stabilizes like our Solar System and is expected to remain that way for billions of years.
“We discovered that the hydrodynamic escape driven by extreme ultraviolet radiation (XUV) offers a plausible explanation for the divergent atmospheric outcomes observed among the four modeled planets, even though they likely formed in similar disk environments with comparable primordial compositions,” the researchers write.
But the authors also point out that there are other mechanisms operating simultaneously and that atmospheric loss through photoevaporation is not the only force that shapes a system. “However, this mechanism is unlikely to operate in isolation or dominate in all cases,” they write. During the pre-main sequence phase, the irradiation is more intensedriving greater mass loss. An exoplanet may also have internal causes for mass loss, such as mass escape driven by waste heat from the planet’s formation. In some cases, sub-Neptunes may continue to experience atmospheric mass loss much later in the solar system’s history.
“Our discovery provides direct observational evidence that the dynamical and atmospheric remodeling of compact planetary systems begins early and offers an empirical basis for models of their long-term evolution,” the authors conclude.
