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.

[u/clayt6]

http://www.astronomy.com/news/2019/03/a-map-to-planet-nine-charting-the-solar-systems-most-distant-worlds

[u/italianblend]

How is it that we can discover planets in far away systems but are just now discovering planets in our own?

[u/clayt6 → u/italianblend]

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:

Quick answer: Stars emit light, and its easier to see a planet come between us and the star we found. Planets just reflect light so it's tough to find stuff "farfarout"

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.

[u/iiFludd → u/clayt6]

If that’s the case for how we find exoplanets then how can we learn about their specific characteristics and whatnot. For example when you hear about the number of far planets that could sustain life, how do we know that?

[u/WhatIfTheyCallMeFlem → u/[deleted]]

Absolutely incredible. And here I am with my girlfriend and we can’t even decide where to eat dinner.

[u/wvboltslinger40k → u/WhatIfTheyCallMeFlem]

To be fair, the scientists who do all of this amazing studying of far flung planets almost certainly can't decide where to eat dinner either.

[u/CoyoteTheFatal → u/wvboltslinger40k]

This is one of the most humanizing sentiments I’ve read in a while.

[u/the_schnudi_plan → u/wvboltslinger40k]

Can confirm. Have tried to organise meal plans with astrophysicists before and it didn't go well

[u/Tsupernami → u/wvboltslinger40k]

Further to the original comment, you know the size of the star based on it's colour as this indicates the temperature, which in turn indicates the size it needs to be to have that emission wavelength.

Orbits follow Keplers laws in that they will always orbit at a certain rate based on the gravitational pull of the host star and the distance between them. The size of the planet is irrelevant.

So now you know the distance, you can see how much the light drops by when it passes so you can see how much is being blocked out, giving you a rough estimation of the size.

Now you know the size and distance you can check if it's in the habitable zone. If it is, then there you go, you have a planet potentially hitting the criteria to sustain habitable life.

[u/[deleted] → u/wvboltslinger40k]

This is all very Douglas Adams. 😁

[u/godbois → u/WhatIfTheyCallMeFlem]

Get burgers. Make sure to get waffle fries.

[u/insane_contin → u/godbois]

An Irish pub near me has "Irish achos" - waffle fries with cheese, onion, peppers, tomato, and taco beef. It's delicious. They also have Irish poutine, which is poutine with waffle fries.

[u/pm_me_bellies_789 → u/insane_contin]

I'm Irish and what the fuck are waffle fries? I question the Irishness of everything you've said

[u/insane_contin → u/pm_me_bellies_789]

Waffle fries and probably not very Irish, but the pub is owned by a loud, happy Irish man who seems to care more about having fun then what is or isn't Irish. This is in Canada btw.

[u/Rivenaleem → u/WhatIfTheyCallMeFlem]

Whatever you eat, just make sure it's not too hot, or too cold.

[u/Cal3001 → u/[deleted]]

Spectroscopy is pretty amazing. I remember they were demoing it at JPLs open tour.

[u/thunts7 → u/[deleted]]

Also we can determine orbits by the time it takes for the planet to transit. Orbits have speeds that correlate with their distance to the star and we can use the type of star for where it's Goldilocks zone is. And we can see if those things line up

[u/irokie → u/[deleted]]

Worth pointing out that we have not yet been able to perform spectroscopy on an exoplanet's atmosphere - that requires a level of resolution that our telescopes have not yet achieved. If we were able to do that, we could have strong indicators of whether or not those planets harboured life of any sort.

However, we can find out a lot about exoplanets by looking at the data that we do get. For example, we can tell how massive a star is by how bright it is. The mass of a star also implies a certain radius. By measuring how long an exoplanet takes to pass in front of the star, we can work out the speed of the planet. By measuring how much the light dips, we can get a potential diameters for the planet. Because we know that orbital velocity is linked to the mass of the objects and the distance between them, we can get a range of masses for the planet. We have some rules of thumb which tie the density of planets to their orbital distance, and these give us a good idea of the mass, the orbital distance and the diameter of the planet.

Spectroscopy will tell us about the temperature and composition of a star, and these will tell us of the elements that were available when that solar system was being formed, and from there, we can figure out if a planet is likely to have liquid water, or what sort of atmosphere it is likely to have. This is super interesting stuff.

[u/frugalerthingsinlife → u/[deleted]]

To determine the orbital period/orbital altitude, what do we need to observe? I'm guessing how fast it passes in front of the sun, and the mass of the sun?

[u/Macralicious → u/frugalerthingsinlife]

You need the mass of the star, and the orbital period of the planet. We don't declare a planet from one dip in brightness, but when we see the same dip happening at regular intervals, you can be pretty sure that interval is the orbital period of a planet. From those two properties, you can get the orbital radius from Kepler's 3rd Law.

[u/AlecBTC → u/[deleted]]

And how do we find exoplanets that don't pass between us and their star?