Why can we see “earth like” planets that are light years away but still debate how many planets are in our own solar system?
In: Technology
kind of apples and oranges here, they’re two very different problems.
Distant stars with planets are only detected by very few methods – one of the most popular in recent years involves the star having a planet that passes between that star and earth – we can measure the difference of light received, and then observe it more for further confirmation & get some other possible details about it’s surface, mass, etc.
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Planets in our own solar system are mostly down to an issue of classification.
The classification of planets for a very long time didn’t have need to be debated because we only “knew of” only so many objects out around Pluto and farther. It hasn’t been until more recent years that we’ve had to discuss the classification to make sure it’s consistent.
These days, that means a planet orbits the sun (1), has sufficient mass to overcome rigid body forces that makes it spherical (2), and has cleared the neighborhood around it’s orbit (3).
That means that, say for our moon, it’s not a planet because it orbits the earth, not the sun.
It means that, for say the asteroid belt, they aren’t massive enough to overcome rigid forces – they aren’t always spherical, and instead oblong or awkwardly shaped because they don’t have enough mass to “pull in” to a spherical shape.
And also the asteroid belt isn’t full of planets because they all share a fairly common orbital path; none of them have cleared it out like Earth has, or something else like Jupiter.
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That then points at Pluto – It does orbit the sun, and it does have enough mass to be more or less round/spherical. But Pluto fails #3 – there’s other objects out in it’s orbit that it hasn’t cleared – Haumea, Makemake, Eris, and a bunch of other kuiper belt objects linger around it’s orbit.
This goes back to what I said at the start – the issue of distant exoplanets isn’t really about calling them a planet or not. It’s more that the only evidence we have of there being a planet points to a large enough object to block enough light from the star that it could only (logically) be a planet.
But with our own solar system, it’s more a matter of how we define the classification of planet – and that didn’t get “updated” until relatively recent years to take into account increasing amounts of data that Pluto isn’t quite like all the other very definite planets.
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