New cameras to probe planets beyond our solar system

A new phase in the study of planets beyond the solar system is about to begin.

Two new cameras designed to directly image Jupiter-class planets orbiting other stars and gather data on extrasolar-planet atmospheres have been brought on line at two of the world's major observatories located on arid mountaintops in Chile's Atacama Desert.

On May 4, the European Southern Observatory lifted the figurative lens cap off of its camera, Sphere, for the first time at its Very Large Telescope array near Paranal, Chile. Meanwhile, astronomers associated with the Gemini Planet Imager (GPI) at the Gemini South Observatory have reported the first science results from their camera, which captured its first light last November. The studies were conducted during GPI's commissioning phase, which is still under way.

Both cameras are slated to undertake the most ambitious direct-imaging campaigns astronomers have yet conducted, surveying up to 600 stars for signs of large, hot, young plants in wide orbits around their host stars. Sphere's search aims to span distances ranging from 5 AU (astronomical units) to 10 AU from their host stars – comparable to the distances between Jupiter and Saturn. GPI is designed to cast a wider net, ranging from 5 AU to 30 AU, the distance from Jupiter to Neptune.

At these orbital distances, planets spend a decade or more completing one orbit. The evidence for their presence is so faint that most common planet-hunting techniques couldn't detect them. Even if they could, astronomers generally want to log multiple detections in order to confidently claim a discovery. Several decades is a long time to wait before shouting “Eureka!”

In addition, "both instruments are going to be doing an awful lot of follow-up observations" to characterize extrasolar planets others have identified, says Peter Michaud, an astronomer and spokesman for the Gemini Observatory, which includes a northern twin to Gemini South perched atop Hawaii's dormant Mauna Kea volcano.

"There's plenty of job security and plenty of work to do for astronomers using instruments like these," he quips.

Although gathered during a portion of GPI's shakedown phase, the first science results from the Gemini camera hint at the potential these new tools have for taking the measure of extrasolar planets.

Exhibit A: Precise measurements of the orbital traits for Beta Pictoris b, a planet with seven times Jupiter's mass orbiting Beta Pictoris, a star 63 light-years away. The planet, which orbits about 9 AU from the star, takes 20.5 years to complete one orbit. It was discovered six years ago via direct imaging by astronomers using the Very Large Telescope array.

In January, astronomers released the "first light" image from the GPI that clearly showed the planet.  One measure of the new camera's capability: It produced a distinct image of the planet with its first 60-second exposure and very little image-processing afterward. Its Gemini South predecessor would have required nearly 4,000 seconds to capture enough light for a useful image – one that also would required heavy processing to yield comparable results.

Using 33 GPI exposures, the team, led by Bruce Macintosh with the Lawrence Livermore National Laboratory in Livermore, Calif., tracked the planet's orbit, providing a more precise measure of the planet's orbital period, distance, and inclination than previous measurements delivered, as well as its relationship to debris discs that still orbit the star.

Of particular interest is the planet's potential for crossing the face of the star, as seen from Earth. Such transits allow astronomers to estimate the planet's size. With its mass known, and with the addition of size, astronomers can estimate the planet's density and hence its bulk composition. No observations have yet been made to try to establish the planet's size.

Based on the new measurements, the planet has a 68 percent chance of transiting between September and December 2017, if it transits at all. The planet's orbital characteristics led the team to conclude that the planet stands only a 4 percent chance of transiting its star.

Exhibit B: A planet orbiting a star, HD 95086, which is 296 light-years away. This planet was discovered two years ago, also via direct imaging by astronomers using the Very Large Telescope array. The planet is about five times as massive as Jupiter and orbits its star at 56 AU.

The GPI not only takes images, but it simultaneously captures near-infrared spectra of the objects it's viewing. This is vital to teasing from the light hot young planets information about the atmosphere's composition.

Using the GPI, astronomers have found that the planet is redder than it should be for its state of evolution. This suggests that the planet has low surface gravity and its atmosphere is dusty.

The results, which have been submitted for publication, appeared as a preprint on the research site arxiv.org.          

Indeed, during April the GPI was applied to a dozen different observation projects as part of the camera's shakedown phase – observations that included Saturn's moon Titan as one of its targets.

The 600-star planet-hunting campaign is to begin at the end of the year if the commissioning phase progresses without a hitch. And while it is a high-profile project, it is only one aspect of a broader agenda astronomers have set for the GPI. Other targets include objects in our own solar system, as well as dust around other stars – either dust in mature systems or in dust discs that may be giving rise to planets.

Indeed, the second brightest feature in our solar system is what is known as zodiacal dust. Dust has provided "some of the prettiest results" so far, Dr. Macintosh says.

"We can trace this dust to see if some unseen planet is shaping it," he says.

Identifying new stars with dust discs will allow other researchers to follow up to see if the dust belts have gaps that would indicate a planet is present or is forming.

And astrophysicists may get a more detailed look at what happens to a star at the end of its life, when it's shedding dust and gas. The same device that allows the camera so see large planets in distant orbits – a device that blocks the light from the star itself – also will allow researchers to observe and measure the faint signatures of dust and gas coming off a dying star.