"We do not 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," said Jennifer Yee of the Harvard-Smithsonian Center for Astrophysics.
The discovery demonstrates that Spitzer -- from its unique perch in space -- can be used to help solve the puzzle of how planets are distributed throughout our flat, spiral-shaped Milky Way galaxy.
Spitzer circles our Sun and is currently about 207 million km away from Earth. When Spitzer watches a microlensing event simultaneously with a telescope on Earth, it sees the star brighten at a different time, due to the large distance between the two telescopes and their unique vantage points.
A microlensing event occurs when one star happens to pass in front of another and its gravity acts as a lens to magnify and brighten the more distant star's light.
If that foreground star happens to have a planet in orbit around it, the planet might cause a blip in the magnification. This technique is generally referred to as parallax.
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"Spitzer is the first space telescope to make a microlens parallax measurement for a planet," Yee added.
In the case of the newfound planet, the duration of the microlensing event happened to be unusually long -- about 150 days. Knowing the distance allowed the scientists to also determine the mass of the planet, which is about half that of Jupiter.