Exoplanets · Space Codex
HR 8799
A young star with four giant planets that telescopes can image as points of light.
Four giant planets photographed directly
Artist's concept. No telescope has seen HR 8799 as more than a point of light, and most exoplanets not even that. What we know comes from how a planet dims or tugs its star. Any picture of its surface, colour or sky, anywhere, is an illustration, not an observation.
At a glance
Key points
JWST found carbon dioxide in them, a sign they formed like Jupiter and Saturn.
- First directly imaged multi-planet system
- Planets glow with leftover formation heat
- JWST saw all four
How they were found
Christian Marois and colleagues imaged the star in infrared with the Keck and Gemini telescopes, using adaptive optics and careful subtraction of the star's glare. Three faint companions appeared at projected distances of 24, 38 and 68 AU, and images taken years apart showed all three moving the same way around the star. A fourth, e, turned up inside the others in 2010. Reprocessing later found b in Hubble images taken in 1998.
Balanced on a knife-edge
Four planets this massive packed this closely should scatter each other within a short time. Many models keep them stable only if their orbits are locked in resonance, possibly a chain in which each planet takes about twice as long as its inner neighbour. Other simulations find stable arrangements without strict resonance, so the system's architecture is still debated.
A scaled-up Solar System
A warm dust belt lies inside the planets and a cold one outside them, broadly like our asteroid and Kuiper belts but much larger. The discoverers described the system as a scaled-up version of the outer Solar System.
Glowing with their own heat
The planets are still hot from formation, about 1,000 K, so they shine in infrared light rather than by reflected starlight. Years of images show them moving along their orbits.
Built like our giants
Carbon dioxide in their spectra means they are enriched in heavy elements, as expected if they grew by first assembling solid cores, the way Jupiter and Saturn are thought to have formed.
Missions
- 2021–
Infrared observatory at Sun–Earth L2, about 1.5 million km from Earth.
Related entries
- Exoplanets: the censusPlanets around other stars
Sources
- Marois et al. 2008, Science 322:1348 (arXiv) (opens in a new tab)
- Marois et al. 2010, Nature 468:1080 (arXiv) (opens in a new tab)
- Goździewski & Migaszewski 2014, MNRAS (arXiv) (opens in a new tab)
- Götberg et al. 2016, A&A 592:A147 (arXiv) (opens in a new tab)
- NASA Exoplanet Archive: HR 8799 overview (opens in a new tab)
- Balmer et al. 2025, AJ 169:209 (arXiv) (opens in a new tab)
- Lafrenière et al. 2009, ApJL (arXiv) (opens in a new tab)
Last reviewed against these sources. Built from public NASA, ESA and JPL data; not affiliated with or endorsed by any agency.