Go out on a cloudless night away from street lights, and you can see thousands of stars from a dark site. For a few minutes, the view is wonderful. Now pick a star and watch it steadily for half an hour...but now what about two hours? What about 50?
We've just started an experiment to look at one star for thousands of hours, and we may be fortunate enough to see the shadows of rings around another world.
Astronomers have detected a large number of exoplanets using the transit method—seeing stars dim by a small amount (typically less than 1 percent) over a few hours when the planet's orbit brings it between us and the star. Since it relies on the light from the star and not the planet itself, this indirect detection technique has been very successful both from the ground in detecting hundreds of transiting exoplanets, and well over a thousand with the Kepler space satellite.
Beta Pictoris (beta Pic for short) is a star in the Southern hemisphere that you can see with the naked eye, forming the faintest corner of a skinny rectangle with the bright star Canopus and the long axis of the Large Magellanic Cloud. The star is relatively young, and has a large circumstellar disk of dust and gas centered on the star, which is almost edge-on to our line of sight from Earth. A large gas giant planet some ten times more massive than Jupiter—beta Pic b—takes on the order of a couple of decades to complete one trip around its parent star.
It's also one of the handful of directly imaged exoplanets we have seen. We have images of the planet as it moves around the star, taken using the largest telescopes in the world. Right now, the planet's almost circular orbit is seen very close to edge on, and from our vantage point it is moving closer to the star. But a graduate student at Berkeley, Jason Wang, analyzed the positions of beta Pic b to see if it might transit its parent star. After a careful analysis, he concluded that the planet will NOT transit in 2017—which makes beta Pic all the more intriguing, since in 1981 something very odd happened.
Beta Pic was originally used for many years as a reference star—one with constant brightness that could calibrate astronomical observations. But in late 1981, the star gradually brightened and faded back to normal over a few weeks, and in a three-night window, its brightness fluctuated by about six percent. Astronomers speculated that a cloud of material from a comet's tail, or maybe a planet with rings, had moved in front of the star. With the later discovery of beta Pic b, the date of 1981 is consistent with a previous passage of beta Pic b in front of the star.
With the later discovery of beta Pic b, it now looks as if circumplanetary material blocked some of the light from the star. We now have a unique opportunity to search for the telltale signatures and composition of clouds and rings around an exoplanet, potentially gaining new insights into planet—and moon—formation.
We may have seen a similar event. Another young star, called J1407, underwent a series of very complex and deep dimmings for over two months in May 2007. These fluctuations were interpreted by our group to be the shadows of a series of giant rings, the largest being over 200 times larger than Saturn's rings, moving between the star and the Earth.
These huge structures can sit within the gravitational domain of a gas giant planet (a region called the Hill sphere), and we think J1407 has a gas giant planet holding these rings in orbit around it. Given a few more million years, though, and gravity will win out and the rings will condense into moons. A similar process is thought to have happened in our Solar system as well.
We think that the same thing was seen in 1981 at beta Pic, and we hope to see something similar happen in 2017 when beta Pic b's Hill sphere passes in front of the star. The problem is that we don't know exactly where any rings or clouds of debris are located, and so we need to watch beta Pic over the whole duration of the Hill sphere transit—which takes over 200 days! Asking for one telescope to look at one star continuously for hundreds of days is highly unusual (the response was typically a polite but firm "no"), but beta Pic is a bright star, so we don't need a large telescope to keep an eye on it.
That's how the "beta Pictoris b Ring" (bRing) project started. At Leiden Observatory in the Netherlands, we've built a small observatory containing two CCD cameras with wide-angle camera lenses that take images of beta Pictoris and the surrounding sky every 12 seconds. In late January, our bRing team installed the observatory at the South African Astronomical Observatory in Sutherland, South Africa, and we are now getting data on beta Pic every clear night it's above the horizon.
A second bRing station is being completed by our team at the University of Rochester and will be shipped to Siding Springs in Australia, where we will keep following beta Pictoris when the sun rises in South Africa. Our goal is to watch for the telltale flickering that indicates a ring or cloud is beginning to cross in front of the star, and we can then trigger observations on much larger telescopes to examine the starlight with spectrographs and look for the tell-tale fingerprints of ices, gases, and dust that make up the intervening material.
We are not alone in looking at beta Pic—two small space satellites will be watching the star closely. The BRITE cubesats have been orbiting the Earth for several years, studying the oscillations of bright stars in order to learn about their inner structure. Dr. Konstanze Zwintz is leading observations of beta Pictoris over 2017 using two of the BRITE satellites. In Paris, Dr. Sylvestre Lacour is preparing for the launch of PICSAT, a cubesat dedicated to monitoring beta Pic over the next year. Others are planning spectroscopic measurements with telescopes big and small, and we will all be looking carefully for that telltale flicker.
Now all we need to do is watch and wait.