- Planet first spotted in 2008; needed confirmation
- Now known to orbit a star 500 light-years from Earth
- A scorching 1,500ºC; still cooling down
A planet only about 8 times the mass of Jupiter has been confirmed orbiting a Sun-like star at over 300 times farther from the star than the Earth is from our Sun.
It is the least-massive planet known to orbit at such a great distance from a star.
The discovery utilised high-resolution adaptive optics technology at the Gemini Observatory to take direct images and light spectra of the planet.
First reported in September 2008 by a team led by David Lafrenière (then at the University of Toronto, now at the University of Montreal and Centre for Research in Astrophysics of Quebec), the suspected planetary system required further observations over time to confirm that the planet and star were indeed moving together through space.
Back in 2008, all astronomers knew for sure was that the young, planetary mass object appeared to be sitting right next to the young Sun-like star.
The closeness of the two objects strongly suggested that they were linked, but it was still possible (though unlikely) that they were unrelated and only aligned by chance.
According to Lafrenière, “Our new observations rule out this chance alignment possibility, and thus confirms that the planet and the star are related to each other.”
With this confirmation the system, known as 1RXS J160929.1-210524 (or 1RXS 1609 for short), provides scientists with a unique specimen that challenges planetary formation theories because of its extreme distance from the star.
“The unlikely locale of this alien world could be telling us that nature has more than one way of making planets,” says co-author Ray Jayawardhana of the University of Toronto.
“Or, it could be hinting at a violent youth when close encounters between newborn planets hurl some siblings out to the hinterlands,” he adds.
First to be directly imaged
With its initial detection by the team using the Gemini Observatory in April of 2008, the object became the first likely planet known to orbit a Sun-like star to be revealed by direct imaging (rather than the indirect through which most such planets have been found).
At the time of its discovery the team also obtained a spectrum of the light from the planet and was able to determine many of its characteristics, which are confirmed in this new work.
Since the initial observations several other worlds have been discovered using direct imaging, including a system of three planets around the star HR 8799 also discovered with Gemini. However, the planets around HR 8799 orbit much closer to their host star.
The team’s recent work on 1RXS 1609 also verified that no additional large planets (between 1-8 Jupiter masses) are present in the system closer to the star.
Future observations should make it possible to measure a very precise velocity of the planet relative to its host star. This will show whether the planet is on a roughly circular orbit, as would be expected if it really formed far from its host star; or whether it is in a very non-circular or even “open” orbit, as could be the case if it formed closer to its star, but was gravitationally “kicked out” following a close encounter with another planet.
Scorching hot planet
The host star is located about 500 light-years away in a group of young stars called the Upper Scorpius association that formed about five million years ago. The original survey studied more than 85 stars in this association.
The planet has an estimated temperature of about 1,500 degrees Celsius and is much hotter than Jupiter, which has a atmospheric cloud-top temperature of about 160 Kelvin (-110 degrees Celsius). The host star has an estimated mass of about 85% that of our Sun.
The young age of the system explains the high temperature of the planet. The contraction of the planet under its own gravity during its formation quickly raised its temperature to thousands of degrees. With this contraction phase over, the planet is slowly cooling down by radiating infrared light (heat). In billions of years, the planet will eventually reach a temperature similar to that of Jupiter.
The observations used the Near-Infrared Imager (NIRI) and the Altair adaptive optics system on the Gemini North telescope. Adaptive optics allows scientists to remove much of the distortions caused by our atmosphere and dramatically sharpen views of space.
“Without adaptive optics, we would simply have been unable to see this planet,” says Lafrenière. “The atmosphere blurs the image of a star so much that it extends over and is much brighter than the image of a faint planet around it, rendering the planet undetectable.”
“Adaptive optics removes this blurring and provides a better view of faint objects very close to stars.”
The Gemini Observatory is an international collaboration with two identical 8-metre telescopes. The Frederick C. Gillett Gemini Telescope is located at Mauna Kea, Hawai’i (Gemini North) and the other telescope at Cerro Pachón in northern Chile (Gemini South).
Together, they provide full coverage of both hemispheres of the sky. Both telescopes incorporate new technologies that allow large, relatively thin mirrors under active control to collect and focus both optical and infrared radiation from space.
Countries in the Gemini partnership are the USA, UK, Canada, Chile, Argentina, Australia and Brazil.
Adapted from information issued by Gemini Observatory / AURA / David Lafrenière (University of Montreal) / Ray Jayawardhana (University of Toronto) / Marten van Kerkwijk (University of Toronto).