Science: Light From Distant Planet Hints at Its Formation

A distant planet named HR 8799c, located 130 light-years outside of our solar system, has water and carbon monoxide—but not methane—in its atmosphere, researchers say. Their findings suggest that a particular planet-forming mechanism, known as core accretion, brought the young planet into existence.

HR 8799c is a gas giant about seven times the mass of Jupiter, orbiting a star known as HR 8799 along with three other planets. Astronomers have been debating whether similar exoplanets (planets found outside our solar system) form via the core accretion process or by another mechanism known as gravitational instability.

Quinn Konopacky from the Dunlap Institute for Astronomy and Astrophysics at the University of Toronto, along with colleagues from Canada and the United States, used data from the Keck Observatory in Hawaii to analyze the light emitted by HR 8799c. Their results, published in the 14 March Science Express, shed light on the formation of this faraway gas giant and provide clues about the formation of our own solar system.

“Our results are most consistent with the planets around HR8799 forming via core accretion, much in the same way we think the planets in our own solar system formed,” explained Konopacky. “By studying the HR 8799 system, we can get a peek at how Jupiter-like planets look very shortly after they form.”

Unlike most other exoplanets, the four planets orbiting HR 8799 were detected directly, meaning that their light was identified separately from that of their host star. This direct detection indicated that HR 8799c was a gas giant, orbiting its star at a distance comparable to Pluto’s distance from our sun.



An artist’s rendering of the planetary system of HR 8799 at an early stage in its evolution, showing the red planet HR 8799c, as well as a disk of gas and dust, and interior planets.
[Image courtesy of Dunlap Institute for Astronomy & Astrophysics; Mediafarm]



“Each time we are able to divide an exoplanet’s light up into even smaller increments, we learn more about the planet’s atmosphere, its composition and its other basic properties,” said Travis Barman, an astronomer at the Lowell Observatory in Flagstaff, Arizona, during a 13 March press teleconference. “This is especially important for the HR 8799 planets because they are so young, providing us with a fantastic opportunity to learn about the formation of planets.”

The analysis by Konopacky and her team provides high-resolution data on the chemistry, gravity and atmosphere of HR 8799c. “The exoplanet has an ideal set of properties, being both fairly bright and located far enough away from the star to allow us to acquire this amazing spectral data,” Konopacky explained.

Two possible mechanisms have been proposed for exoplanet formation: a multi-step, core accretion process by which gas slowly accumulates onto a planetary core, or a process known as gravitational instability that involves the simultaneous creation of a planet’s interior and atmosphere.

A clue to the exoplanet’s formation lies in the fact that HR 8799c’s atmosphere contains slightly elevated levels of carbon compared to oxygen. The researchers suggest that grains of water ice must have condensed in the planetary disk surrounding HR 8799 and depleted the oxygen in the atmosphere.

“These ice grains stuck together to make bigger ice chunks, a few kilometers across, that kept colliding and building up the planet’s solid core,” Konopacky suggested. “The atmosphere came later—from gas that the planet attracted after it got big enough.”

These findings imply that a core accretion process, similar to the one that shaped our solar system—with gas giants far away from the sun and rocky planets closer to it—also led to the formation of HR 8799c.

“In many ways, the HR 8799 system looks like a ‘scaled-up’ version of our solar system,” Konopacky explained during the teleconference. “It has four distant gas planets in addition to what looks like the asteroid belt interior to the closest gas giant and something like an Oort cloud beyond the most distant gas giant.” (The Oort cloud is theorized to be an icy, spherical cloud of debris located approximately one light year from the sun.)

“So not only are we really excited that we have this tantalizing evidence of the way another planetary system may have formed,” she added, “but we can even speculate that maybe there could be planets more like the Earth—terrestrial planets—closer in to the star HR 8799.”


Read the abstract, “Detection of Carbon Monoxide and Water Absorption Lines in an Exoplanet Atmosphere,” by Quinn Konopacky et al.

Listen to a 13 March press teleconference about the Science paper.