A new program will let amateur astronomers detect exoplanets - worlds outside our solar system - by observing nearby bright stars and recording faint dips in their brightness caused by transits from planets in orbit around them.

The program is called Open Source Differential Photometry Code for Amateur Astronomy Research (OSCAAR) and was developed in part with NASA funding. It is available for free online.

"From NASA's Kepler mission, we know there are potentially thousands of exoplanets or more," said Brett Morris, a research associate at NASA's Goddard Space Flight Center in Greenbelt, Md., who is lead developer of the OSCAAR program.

"These planet candidates were discovered by looking at the brightness of thousands of stars over time. A certain fraction of the planets orbiting those stars are aligned such that they transit the star -- that is, they pass in front of the star as seen from earth. The transit will block out just a small amount of that star's light when we view it from Earth. If we measure that star's brightness over time, it will change by up to two or three percent, which can be measured by the commercial-grade detectors that many amateur astronomers and small observatories at academic institutions already have."

Kepler has a clear view from its location in space in orbit around the Sun, far from the distortion produced by Earth's atmosphere. However on Earth, atmospheric variability, such as thin clouds passing overhead, make measuring the brightness changes due to planetary transits more complicated, according to Morris.

"The purpose of a differential photometry code - the differential part - is to compare the changes in brightness of one star to another nearby. That way you can remove changes in stellar brightness due to the Earth's atmosphere. Our program measures the brightness change of all the stars in the telescope's field of view simultaneously, so you can pull out the change in brightness that you see from the planet-hosting star due to the transit event."

Amateurs who would like to try the program need a telescope equipped with an electronic light detector, called a charge-coupled device, or CCD, and software capable of reading the output from the CCD with a computer running one of the following operating systems: Windows 7 and up, Mac OS X 10.6 and higher, Ubuntu 12 and up or similar Linux distribution.

"We're not saying the program will give groundbreaking results or science competitive with Kepler, unless you adapt OSCAAR for that purpose," said Morris. "But the observations can be very satisfying knowing that you're watching other planets, and we hope that OSCAAR users will be inspired to take their exoplanet studies further after they get a taste for photometry."

According to Morris, people will detect mainly hot-Jupiter type exoplanets around nearby stars. These are large planets, so the transit will be substantial enough to be detectable. Also, hot Jupiters move quickly since they orbit close to their parent stars, so the transit will proceed rapidly enough to be seen during an observing session of about six hours or so on a typical night.

The number of stars people could search using the program depends on the quality of their equipment and the viewing conditions at their location -- the darker the night sky, the more stars they will be able to analyze. "People should be able to make measurements of maybe about a dozen bright planet-hosting stars even from urban areas with heavily light-polluted skies," said Morris.

"We've successfully used the program at the University of Maryland Observatory in College Park, which is located within the beltway of Washington D.C., and we have pretty miserable light pollution compared to some of the big professional observatories," says Morris.

Morris hopes astronomy students and educators will use the program. "With the Kepler mission, exoplanets have become a hot topic in astronomy," said Morris. "More and more students are interested in doing their own observations of them, but there's a shortage of mentors who have experience doing that because it's a newer sub-field in astronomy. Our program is meant partly to help fill that gap by putting a tool out there that has lots of tutorials so undergraduates can pick it up on their own."

The team also encourages people to modify the program. "With some straightforward adaptations to this code, it can be applied to observations of some other astrophysical phenomena," said Morris.

"Anything in the sky that changes brightness over time could be observed with OSCAAR. For example, variable stars can become a few times brighter in just one night, a change many times greater than what is seen from a star with a transiting exoplanet. Alternatively, some asteroids are irregularly shaped and rotating, so the cross-section seen from Earth gets bigger and smaller, causing the amount of light from the asteroid to increase and decrease, which can be detected with OSCAAR. We've tested the program in these situations and it works pretty well, but there are always slight tweaks that would make it work even better, so we encourage people to make them."

"We're hosting the program on 'GitHub', a website for collaborative code development, so it's easy for people to make a copy of the code, modify it, and offer it back to the user community," adds Morris.

"Also, we're issuing the program under the 'MIT license' for open-source code, one of the most liberal licenses you can put on code so that other people can use it for whatever they want. They can remix the code, adapt it into other works they've already created or create new things around it, and even use it in their own proprietary software."

The team includes Morris, Daniel Galdi, and Dharmatej Mikkilineni of the University of Maryland and Luuk Visser of Lieden University/Technical University, Delft, the Netherlands. Development of the program was funded in part by the NASA Astrophysical Data Analysis program.