Kepler found most of its planets within this small scope of space (Credit: Jon Lomberg)

It's a well-known fact that our galaxy is astoundingly large, containing between 200 and 400 billion stars, nearly twice as many planets, and who knows how many minor planets and moons. However, despite its size, we can only see a tiny fraction of the Milky Way; In fact, the most distant star we can resolve with the naked eye is just 4,000 light-years from Earth. Kepler—the revolutionary tool credited with discovering over 1,000 planets beyond our solar system—suffers from similar constraints, it mostly searches for planets in a small cone only 3,000 light-years in size.

Astronomers use different methods to find candidate planets a bit farther out. Namely, they combine a few different elements, each focusing on specific tasks. Keeping in this spirit, researchers from the Harvard-Smithsonian Center for Astrophysics—working in conjunction with a team from OGLE, or the Polish Optical Gravitational Lensing Experiment—have reportedly discovered one of the most distant exoplanets yet: a gas giant that lurks around 13,000 light-years away from Earth.

In order to make this discovery, the team employed the use of NASA's Spitzer Space Observatory and the Warsaw Telescope at the Las Campanas Observatory. The latter was specifically used to monitor microlensing—a celestial phenomenon that sees light become distorted by an object's gravity. Basically, when stars or galaxies appear to overlap with background objects, the distortion caused by the warping of spacetime essentially magnifies the farthest of the two sources, helping us study a distant object more closely.

This illustration shows how exoplanetary microlensing works (Credit: NASA)

For exoplanets, when microlensing is in effect, planets orbiting the frontmost source might leave a discernible signature on the magnified object, thus allowing us to deduce the existence of planets that are much too distant to be seen traditionally. Overall, around 30 planets have been detected using this method, the most distant of which is believed to be 25,000 light-years away. Most were found within the Milky Way's central bulge, as it provides more microlensing opportunities than more sparsely populated regions do.

This method has one rather large setback: as a whole, astronomers have a difficult time determining just how far away magnified objects actually are, but Spitzer provides an easier work around. From the Harvard-Smithsonian Center for Astrophysics:

In the coming months, the teams plan to observe around 120 microlensing events with Spitzer, OGLE and perhaps other ground-based observatories. In any case, even if no other planets are found, researchers hope their work can pave the way to a better understanding of how planets are scattered across our galaxy. "We don't know if planets are more common in our galaxy's central bulge or the disk of the galaxy, which is why these observations are so important," Jennifer Yee—a scientist from the Harvard-Smithsonian Center for Astrophysics (CfA), and the lead author of the paper—notes.

Moreover, "Now we can use these single lenses to do statistics on planets as a whole and learn about their distribution in the galaxy," another researcher remarked.

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