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.
“Seeing” Earth like plants isn’t exactly accurate. Usually it’s more like calculating the probability of it’s presence.
The farther out from the star it becomes harder to detect.
99% of extrasolar planets (we don’t actually know what percentage, but most) can’t really be detected by us. Certainly not *directly* by looking at them.
Only seeing 8 out of 9 of the planets in our solar system is well within reason, with that fact in mind.
Our only reliable way of detecting exoplanets currently is by recording the periodic dimming of the star as the planet passes in front. This mean that the ecliptic plane of the target system has to alight with our line of sight, which does not occur every time. So basically for every exoplanet we find there are possible millions we don’t see in the same area.
As to our solar system, some of the debate is about the classification of planets which doesn’t involved knowing if a planet is there or not. But yes some debate on the existence of planets further away, so how is this unreserved. Well consider the inverse square law: the light from the suns dims by the square of the distance to it. So for objects beyond Pluto or Neptune, the light hitting them get very small. If those objects are small in size, the effect is compounded. Now consider that light has to bounce off the planet (only a small fraction is reflected) and come back to earth, and that light too follows the inverse square law. By the time this light reaches the earth it is dimmer than far away stars and gets lost in the background noise so they are near impossible to observe. Their existence is theorized as an explanation for small perturbations is the orbits of known bodies which we are able to measure with some precision.
The are two main ways for us to see planets in other systems:
If a plant passes directly between us and it’s start, we can see star getting a little bit dimmer. We can get some information about the planet from how much light the planet blocks.
Second method is to see how much start wobbles as a planet tags on it via gravity while it’s going around the star. Once again we can get some information about the planet from its star’s movement.
Those methods don’t work for very small planets that don’t block enough light for us to detect, and also for planets who’s orbit might be so huge that we cannot see their tag on the star. Same reasons apply to our own system, if planet is very far away from the sun, it will be too dim for us to notice without knowing where to look, and we will not see any gravitational effects from it unless it gets closer.
I have pretty basic knowledge in this field from personal research because I was curious, so I would appreciate if anyone with better understanding would add any corrections or details.
Your question is based on a faulty premise becaue we *don’t* debate how many planets are in our own solar system.
We debate what the *definition* of a planet is. As the definition evolved what “counts” as a planet changes.
Presumably you’re referring to Pluto. For many years, we considered it a planet, until astronomers and astrophysicists got more precise with their definition of a planet. With the more narrow definition, Pluto no longer fit the definition of a planet, so it was recategorized as a dwarf planet.
None of that has anything at all to do with being able to detect planets around other stars.
The debate over how many planets we have in our solar system comes from our inability to agree on what makes it a planet. As we improve our telescope technology we’ve discovered more objects orbiting the Sun. Some of those objects are bigger than what we would consider an asteroid, but smaller than what we traditionally think of as a planet.
https://solarsystem.nasa.gov/planets/in-depth/
We can detect Earth like planets not see them, we can work out the approximate distance from the star and the mass of the planet and work out what conditions might be like. https://youtu.be/AnYye_c8rI4
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