# What would we see if we looked at our Solar system from 2,000 light years away with our current technology?

Assuming the tables were turned and we would live in a system like Kepler-422/423/424, some 2,000 ly away. If we'd look at the Solar system with a telescope like Kepler and using techniques like radial velocity and/or transit photometry, what would we see? Would it be possible to identify any planets at all, considering the larger ones like Jupiter and Saturn have pretty long orbital periods and the smaller ones are, well, just too small?

edit: Obviously I didn't account for the possibility of misaligned declination, thanks Jim for pointing this out. Now, if we would look head on, e.g. be in a position to theoretically "see" the planets, would we also detect them in the data?

edit 2 (this does not answer the question, so I put it up as an edit for some additional information): Jim's comment to the question (about declination) made me look up if there was a possibility to also give some kind of probability to whether planets are in our line-of-sight, i.e., statistically, how many orbital planes would we look at from the right angle, so that transits could be seen in the first place?

Obviously, the probability to detect planets transiting their host star depends on their vicinity to the star. Closer planets have a higher probability to be found than those more far out. For Earth and Venus the probability to be found transiting from an observation point anywhere (360 degrees) in space would basically be 0.47% and 0.65% respectively. These probabilities of course change with input of more variables.

• We'd probably be able to detect the Gas Giants, given the distance, if we catch them transiting the sun at that time. Excellent question!
– user60063
Oct 2, 2014 at 14:33
• All of the Kepler systems you listed have declinations around 45 degrees. So you wouldn't be able to see a transit of any of the planets in our solar system from those stars
– Jim
Oct 2, 2014 at 14:53
• I disagree with Omen (even if the declination was not so great) because of the large distance between the gas giants and the Sun. It's much easier to detect gas giants around other stars because a) many are larger than the gas giants in our solar system, and b) most are much closer to their home stars. Oct 2, 2014 at 14:57

I did some research and I think I can answer my question myself now, after all. I hope you find it interesting.

As it turns out, the technology of the Kepler space telescope would indeed allow detection of all Solar system planets except Mercury and probably Mars, i.e. all of them are big enough to be seen by it from a distance of about 2,000 ly. However, that doesn't mean that we would also be able to detect (=scientifically confirm) all of them, even if we would look at the planets' plane head on.

The reason for this is the long time we would need to gather the data. Kepler observes transits of planets in front of their sun. To get a planet confirmed there's more than just one transit observation needed. So if we look at Jupiter (large enough to be detected), with an orbital period of twelve years it would take 36 years of data to confirm its existence by this measure/method. Even harder: Saturn, Uranus, Neptune with orbital periods of 29, 84, and 165y, resp. However, Kepler would be able to detect and confirm Earth and Venus with about 3 years of data. That Kepler has found so many large planets so far has actually nothing to do with their size but with their relatively short orbital periods.

So the answer is: Yes, if we would look at the Solar system with the Kepler space telescope from 2,000 ly away and from an angle to see the planets transit the Sun, we would have been able to confirm Venus and Earth with the transit observation method. Maybe we would have seen one or the other transit by one of the four gas giants but not enough of them. However it would still be possible to use the data to try to confirm them with alternative methods (such as radial velocity observations) from other, even groundbased, telescopes.

That said, as the Kepler telescope has now stopped gathering further data and has been operating for about four and a half years it's quite possible that we find some Earth and Venus sized planets if their orbital periods are not much longer than 1, maybe 2 years. And while the transit method might not lend itself to easily detect even gas giants if they are too far out, they might be identified with other methods once everyone zooms in on the most interesting planetary systems.

• As Earth and Venus have orbital inclinations that differ by 3.39 degrees it would not be possible to see both the Earth and Venus through transit observations. I think it would be a stretch to detect Mercury too. Jupiter would be detectable using current doppler methods/technology and probably Saturn too if you observed for decades. Dec 20, 2014 at 22:21

Well the light which is reaching there now, the light which is being detected by a telescope set up in the Kepler planets was emitted from the Sun's solar system 2000 years ago since light took 2000 years from here to reach Kepler planets. So any event which occurred 2000 years ago in our solar system will be visible now in the Keplers.

If we are (somehow) able to look at that solar system then we might be seeing some storms or solar flares from Sun or anything which might have happened in almost 14 AD (2014 minus 2000).

By the way relative to the old space and its galaxies which formed billions of years ago there may not have been any significant change in just 2000 years!!!

• To add to this answer, we wouldn't be able to resolve any events happening on Earth. It would take us a few years just to determine that Earth exists
– Jim
Oct 2, 2014 at 19:04