JWST captures four exoplanets orbiting an alien star in a single system

In the vast expanse of the cosmos, there exists a remarkable celestial system known as HR 8799, located approximately 133 light-years away from Earth. 

This artist’s rendering shows an alien star HR 8799 and planets orbiting it.

This enigmatic system has recently captured the attention of the astronomical community due to its unique characteristics and the invaluable insights it offers into the process of planet formation. In this comprehensive article, we delve deep into the intricacies of HR 8799, examining its stellar host, the four massive exoplanets that call it home, and the groundbreaking observations made possible by NASA's James Webb Space Telescope (JWST), specifically its Mid-Infrared Instrument (MIRI) coronagraph.

The HR 8799 System: A Stellar Gem

HR 8799, a star 1.5 times more massive than our Sun and nearly five times as luminous, is at the heart of this captivating system. What sets HR 8799 apart is not just its star, but the presence of four colossal exoplanets orbiting it. These planets, named HR 8799 b, c, d, and e, are massive giants, each boasting a mass between 5.7 and 9.1 times that of Jupiter. This places them precariously close to the boundary where deuterium fusion occurs, a defining characteristic of brown dwarfs. Their orbits range from 16 to 71 astronomical units (AU) from the star, with orbital periods spanning from 45 to 460 years. Furthermore, these planets have radii approximately equivalent to 1.2 times that of Jupiter.

The Significance of HR 8799

The HR 8799 system holds significant importance in the realm of astrophysics for several reasons. First and foremost, the existence of massive planets on such wide orbits is a rarity in our universe. This rarity makes the system a treasure trove of information for scientists eager to understand the complexities of planet formation and the dynamics of young solar systems. Notably, HR 8799 is a relatively young system, only around 30 million years old, which further enhances its appeal for astronomers seeking insights into the early stages of planetary evolution.

The Role of JWST's MIRI Coronagraph

The JWST, equipped with its formidable MIRI coronagraph, played a pivotal role in unraveling the mysteries of HR 8799. The MIRI instrument, designed for high-contrast imaging, offered astronomers the capability to peer into the heart of the system with unprecedented clarity. By employing this cutting-edge technology, scientists aimed to characterize the planetary atmospheres and address lingering questions about the nature of these massive exoplanets.

This is one of the JWST’s MIRI images of HR 8799 and its four planets. Credit: Boccaletti et al. 2023

What Did JWST Discover?

The MIRI images of the HR 8799 system presented a new perspective, distinct from previous observations in the near-infrared spectrum. These images not only confirmed the presence of the four exoplanets but also revealed localized, extended central emissions. This breakthrough allowed astronomers to refine their understanding of the system's key attributes.

One significant revelation from the JWST observations was the confirmation that the four giant planets in HR 8799 are not brown dwarfs, as previously speculated. Their distinctive colors, as indicated by the data, unequivocally distinguish them from field brown dwarfs. Additionally, temperature measurements of these planets, ranging from 900 K to 1300 K, highlighted intriguing variations. HR 8799 b, the faintest among them, exhibited a lower temperature than previously assumed, showcasing the remarkable precision of the JWST's measurements.

The powerful filters on the JWST’s MIRI instrument ended the debate about a potential fifth planet at HR 8799. This MIRI image helped determine that the object is in fact a background object. Credit: Boccaletti et al. 2023

Furthermore, MIRI identified two atmospheric chemicals with certainty: H2O and CO. While methane detection remained a subject of debate, the mere presence of this potential evidence aligned with the hypothesis that these celestial objects are indeed planets, not brown dwarfs. In the realm of celestial objects with these temperatures, brown dwarfs consistently exhibit the signature of methane, making this discovery highly indicative of planetary characteristics.

The Quest for Ammonia and a Background Mystery

Among the myriad insights gained from the JWST observations, the search for ammonia, a solid biosignature on terrestrial planets, remained a challenging endeavor. Unfortunately, the elevated temperatures of HR 8799's planets rendered ammonia undetectable with the available data. The potential detection of ammonia would have been groundbreaking news, but the system had other revelations in store.

The HR 8799 system's enigma extends beyond its planets, as it is also home to a distinctive debris disk. This disk, notable for having two belts, previously fueled debates about its origin. Some speculated that an unseen fifth planet or a dust clump might be responsible for the inner edge of the outer belt. However, the JWST's meticulous observations conclusively identified this feature as a background object, thereby resolving the debate.

A New Era of Exploration

In closing, the JWST's encounter with HR 8799 marks a monumental milestone in our quest to understand the complexities of young exoplanetary systems. The MIRI instrument's high-contrast imaging capabilities have opened an exciting new frontier for astronomers, enabling the characterization of systems like HR 8799 with unparalleled precision. While challenges persist, and room for improvement remains, these pioneering observations will undoubtedly pave the way for even more remarkable discoveries in the future.

As we continue to unravel the mysteries of the cosmos, HR 8799 stands as a testament to the boundless potential of human exploration and our relentless pursuit of knowledge in the infinite expanse of space.

Research paper