One recent find, however, in the HR 8799 system does bear some similarity to the solar system and highlights advances in an important planet-finding method. Of the 500+ planets in the exoplanet encyclopedia, most were discovered using either the radial velocity or transit techniques. This current planet was found by directly detecting the light emitted by each planet in that system.
Unlike the 4.6 billion-year-old Sun, the star and planets in the HR 8799 system are a youthful 30–100 million years old. This system just finished the process of formation. Previously, observations of HR 8799 revealed three Jupiter-class planets orbiting with distances of 24, 38, and 64 astronomical units (AU) from their star, respectively. One AU is the distance from the Earth to the Sun. Jupiter orbits at 5 AU; the newly discovered planet orbits at 14 AU.1
The HR 8799 system contains four gas giants on roughly circular orbits and bears some resemblance to our solar system; however, striking differences also exist. Besides its youth, HR 8799 also contains 1.5 times the mass of the Sun. The extra mass may partially explain why all the HR 8799 gas giants are more massive than Jupiter whereas all the solar system gas giants are less massive (except for Jupiter, of course). Furthermore, the closest gas giant in the HR 8799 system would orbit in between Saturn and Uranus. The furthest is more than twice as far as Neptune. Also, the debris disk—analogous to the Kuiper Belt—around HR 8799 dwarfs the one in our solar system as seen in the image below.
Although this system may harbor smaller rocky planets closer into the star, scientific interest in HR 8799 arises mainly because of the difficulty explaining how it formed. Astronomers have two models for gas giant planet formation: (1) core accretion and (2) gravitational collapse. These two models together can account for planets like those in HR 8799, but neither model by itself can explain the size and location of all four gas giants.
This planetary discovery, and others to come in the future, will help astronomers gain a better understanding of how planets form and move around their host stars. Just as past finds provided new reasons to believe that a Creator fashioned Earth for humanity, RTB expects these future advances to add even more reasons.
- Christian Marois et al., “Images of a Fourth Planet Orbiting HR 8799,” Nature 468 (December 23, 2010): 1080–83.