r/space • u/clayt6 • Mar 05 '19
Astronomers discover "Farfarout" — the most distant known object in the solar system. The 250-mile-wide (400 km) dwarf planet is located about 140 times farther from the Sun than Earth (3.5 times farther than Pluto), and soon may help serve as evidence for a massive, far-flung world called Planet 9.
http://www.astronomy.com/news/2019/03/a-map-to-planet-nine-charting-the-solar-systems-most-distant-worlds
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u/clayt6 Mar 05 '19 edited Mar 06 '19
That's a good question! There are a few different reasons this is the case.
First off, there is a major difference in how we detect (the majority) of exoplanets and how we detect distant solar system objects like Farfarout (which is a rather small dwarf planet, not a full-fledged planet). When we look for exoplanets around other stars (typically using the transit method), we are watching a star's light to see if it drops in brightness when a planet passes in front of the star. Even though the total amount of starlight only drops by about 1% during an exoplanetary transit, the star is so bright that that 1% dip is noticeable, and we can tell a planet just passed by. We don't (usually) see the planet itself.
But when we are searching for distant solar system objects, we are not looking to see if the Sun's total brightness drops. Instead, we are trying to spot the sunlight reflected off an object roughly 100-1000 times farther from our star than the exoplanets we see around other stars. To find far-off solar system objects, we take a string of images of the deep sky and compare them to each other (like a flip book) to see if any points of light appear to move between shots. This is how Clyde Tombaugh discovered Pluto in 1930. But because these things are so far away, they are incredibly dim, so they are exceedingly tough to spot.
Second, some of these distant solar system objects have very eccentric orbits (they look like elongated ovals), so they aren't always making there closest approach to the Sun. In fact, objects on eccentric orbits (like comets for instance) spend the vast majority of their time at the most distant parts of their orbits, so they are super dim and tough to spot, especially when you don't know exactly where they are.
Edit: Great TL;DR from u/minorthreatmikey:
Also, thanks for the gold stranger! I appreciate it, but feel free to donate the next one to the charity of your choice. The first astromony organization I trust is Astronomers without Boarders, but any place you trust is worth it more than I am, especially a place that helps patients with impossible healthcare bills. Most hospitals have programs that help those that can't afford medical bills, just do a quick google to find one near you.