An international team of astronomers has determined the masses of four nascent exoplanets orbiting the young star V1298 Tau. The findings, published in the journal Nature, provide direct observational evidence for the early evolutionary stages of super-Earths and sub-Neptunes, planet types prevalent throughout the galaxy but absent from our Solar System. This research aligns with theories that these planets initially possess expanded, low-density atmospheres that are subsequently lost, leading to their contraction into denser, more compact forms.
Context of Exoplanet Research
Observations have established that planets larger than Earth but smaller than Neptune orbit a majority of stars in the Milky Way galaxy. The absence of such planets in our Solar System has presented a challenge in fully understanding their formation and evolution processes. Prior to this study, astronomers had theorized mechanisms for this planetary growth based on observations of large young planets.
The V1298 Tau System
The V1298 Tau system, located approximately 352 light-years away in the constellation Taurus, hosts a star estimated to be about 20 million years old. This age is significantly younger compared to the Sun's 4.5 billion years. Orbiting this young star are four exoplanets, which are currently in a phase of rapid evolution. Professor Erik Petigura from the University of California characterized the V1298 Tau system as an observational link connecting star- and planet-forming nebulae with mature planetary systems that have been discovered. The system is considered a potential precursor for the compact, multi-planet systems frequently observed across the galaxy.
Planetary Characteristics and Evolution
The observed exoplanets currently exhibit radii five to ten times that of Earth and masses five to fifteen times Earth's mass. This results in a low density, comparable to that of polystyrene foam. This low density indicates expanded atmospheres, a condition attributed to the heat and radiation emanating from their young host star.
This atmospheric expansion is predicted to lead to significant gas loss into space. According to Professor James Owen of Imperial College London, the planets have experienced substantial atmospheric loss and cooled more rapidly than predicted by some standard models. He stated that their evolution is ongoing, with further atmospheric depletion and contraction expected over billions of years, which would transform them into compact super-Earth and sub-Neptune systems. Trevor David of the Flatiron Institute, who led the system's initial discovery in 2019, noted that while the large radii of young planets suggested low densities, these characteristics had not been directly measured. He stated that the mass measurements provide the first observational confirmation of these low densities, offering a benchmark for planetary evolution theories.
Observational Methodology
The determination of these planetary characteristics involved a decade of observational data collected from various global and space-based telescopes, including Japan's NAOJ 188-cm telescope in Okayama. Researchers examined the planets by observing their transits, which are brief reductions in the star's brightness as a planet passes in front of it. These transit events typically involve a reduction of approximately 1% of the star's light.
Transit depth provides data on a planet's radius, and timing indicates orbital characteristics. Initial observations indicated larger planetary sizes, but transits for two outer planets were initially undetected, creating uncertainty regarding their orbits. Researchers employed computer models to refine orbital possibilities. Subsequent observations with ground-based telescopes successfully identified the planets.
Once all four orbits were established, a detailed analysis was conducted to determine the planets' masses. Astronomers detected minor variations in the planets' orbital timings, known as transit timing variations (TTVs). These variations are attributed to the gravitational interactions between the planets, which cause slight accelerations or decelerations in their transit timings. The magnitude of these timing changes correlates with planetary mass, enabling the team to measure the planets' masses for the first time.
Implications for Planet Formation
The observed low density of these nascent planets addresses aspects of planet formation models. Standard models predict that planets that simply form and cool would typically be more compact. The low density observed suggests a rapid transformation involving substantial atmospheric loss and cooling. The V1298 Tau system now functions as a research environment for understanding the origins of the most common planetary systems in the Milky Way, providing insights into the early evolutionary stages of young planetary bodies. Data from systems like V1298 Tau may also contribute to understanding the reasons for the absence of super-Earths and sub-Neptunes in our own Solar System.