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October 17, 2012 10:26 AM   Subscribe

Could Alpha Centaurians detect planets around Sol?

Given Alpha Centaurian have a similar level of telescope technology that we do, could they detect any of the planets around our sun? Most of the planent detection technology I've heard about would miss our tiny inner planets since they don't exert much pull on the sun. The would also miss our outer planets as they have huge orbital periods. They might be able to detect a transit but they would never be able to confirm it without waiting 12 years (Jupiter's orbital period).

However, I'm not an astronomer and I can't keep up with all the fancy gadgets. Could any of our technology in the hands of Alpha Centaurians detect any of our planetary bodies?
posted by chairface to Science & Nature (13 answers total) 6 users marked this as a favorite
Given that there has just been an announcement of the discovery of a planet in the Alpha Centauri system, probably yes.
posted by Logophiliac at 10:39 AM on October 17, 2012

No, for an inobvious reason: the surface temperature on that planet is so hot that the sensitive photo detectors* required to make the images that detect Sol's gravitational wobble would be flooded with noise, making the wobble impossible.

*In fact, few known detector technologies would even function at the 400F+ surface temp I saw as an estimate for Alpha Centauri-1.
posted by IAmBroom at 10:42 AM on October 17, 2012 [3 favorites]

Best answer: Yes.
posted by the Real Dan at 10:45 AM on October 17, 2012 [1 favorite]

No, we do not yet have the level of technology to detect small rocky planets at a remove from the star - our astronomers can only detect close-in rocky planets of roughly earth size or larger, or large gas giants further out.

The technology to detect earth-like rocky planets in the "goldilocks" zone - suitable for advanced life, neither too hot nor too cold - is not very far off, though. A great deal of it is simply information processing - as computers get more powerful, they're able to extract more information from the data we've collected. Also, more powerful telescopes are on the way, to provide a richer source of raw data. I believe we'll find one in the next decade.
posted by Slap*Happy at 10:49 AM on October 17, 2012

For those answering, chairface is asking if they could detect any planets looking from their system to ours.

Chairface is not asking if they could detect Earth. Not asking if they could detect planets from the surface of the recently discovered hot planet (Alpha Centauri Bb).

From their solar system to ours, with our level of technology. That seems to be the question. And despite my slightly facetious answer above, I think the answer is yes. They could pick up our radio waves if they were looking for them.
posted by hamandcheese at 10:55 AM on October 17, 2012 [5 favorites]

Response by poster: Thanks for the clarification, hamandcheese. Radio wave detection isn't really the question -- a sufficiently advanced culture can emit radio waves without an attached planet. We do this. Detecting Voyager's signals would tell you there's intelligent life but not if there's any planet still hanging around.

the Real Dan's answer does appear to say that yes, Jupiter is detectable. The paper cited dates to 2003 so I would assume our hypothetical neighbors would be even more advanced by now and Jupiter would be pretty easy to spot.

Logophiliac - planets are detectable based on the observers technology and the planets in question. Nearly every planet detected so far has been extreme in some way. Usually they orbit crazy close the host star or are giants. We don't have any planets that meet those criteria. That's why the question isn't so simple to answer.

IAmBroom - I didn't ask about the new planet. I meant any detector anywhere near AC. There may be earth like planets just over there but we don't have the technology to detect them.

So let me restate the the question.

Can someone in the vicinity of Alpha Centauri detect any of Sol's planets using technology similar to what we have today (solar wobble, transits, etc)? Bonus follow up: Is Jupiter the only detectable body and would it take at least 12 years of observations to detect?
posted by chairface at 11:27 AM on October 17, 2012

This article in the New York Times explains the relationship between an exoplanet's proximity to the star, relative size and the size of the 'wobble' signal.

In a nutshell, the discovery pushed this technique to its current limit using available technology. Whether this is a fundamental limit is anoter question.
posted by TheOtherGuy at 11:36 AM on October 17, 2012

Best answer: Jupiter is detectable - it induces a Doppler Wobble in the Sun of 12.4 m/s. By comparison, these results were for wobbles something like 0.5 m/s. No other planets in the Solar System are above the 0.5 m/s threshold.

Do you need 12 years? Well, it depends on how much data you want. The current standard is usually at least 2 full orbits before declaring a "detection." So, that'd be 24 years. However, you could, in theory anyway, see something in much shorter time. So, 1/4 of an orbital time? 1/2 an orbital time? It depends on how much you think you've accounted for all of the systematic variation in measured radial velocities. Most Earth-bound astronomers will be suspicious if you haven't seen at least one orbit, I think.
posted by Betelgeuse at 11:47 AM on October 17, 2012 [2 favorites]

Oh. And transit is much easier. With current technology, Alpha Centaurians could easily see Earth's signature if they were aligned just right.
posted by Betelgeuse at 11:49 AM on October 17, 2012

Alpha Centaurians could easily see Earth's signature if they were aligned just right.

They aren't. Transits would only be visible to star systems on or very near our ecliptic (roughly, the plane in which the planets in our Solar System orbit the sun).

Alpha Centauri is about 40° away from the ecliptic, so they won't be able to see our transits. (Navigational Star Chart; the ecliptic is the sinusoidal dashed line; Alpha Centauri on this map is the star labeled "38 Rigil Kentaurus" at about 60°S declination and 140° SHA.)
posted by DevilsAdvocate at 12:41 PM on October 17, 2012 [5 favorites]

Response by poster: Thanks everyone! I'll mark this as resolved. It sounds like our doppelgängers could detect Jupiter with sufficient patience or luck but the rest of our system would remain mysterious.

It's exciting to know that and and reflect just how much stuff we must be missing, given what we have already detected.
posted by chairface at 1:38 PM on October 17, 2012 [2 favorites]

To go into a little more detail, if they are relying on the doppler wobble method, it is easy to figure out the speed of the wobble. Formula is orbital speed in m/s X mass (in terms of solar mass, solar mass=1). So for some of the solar system planets we have:

* Jupiter, 12.5 m/s
* Saturn, 2.8 m/s
* Earth, 0.09 m/s
* Venus, 0.09 m/s
* Mercury, 0.008 m/s
* Mars, 0.008 m/s

The others are even smaller--I haven't bothered to calculate them.

Dan's link mentions the current limit of detection at about 3 m/s. But it's a rather old article, and according to the NYT article the recent detection is at about 20 inches/second or 0.5 m/s.

So that appears to be about the current limit.

That puts Jupiter and Saturn both well above the currently detectable limit, Earth and Venus just below it, and Mercury and Mars way below it.
posted by flug at 6:16 PM on October 17, 2012 [1 favorite]

Since Alpha Centauri is not on the ecliptic, as DevilsAdvocate points out, it means that Jupiter's orbit is not edge on when viewed from there. This reduces the Doppler wobble to about 77% assuming a 40 degree angle. It should still be easily detectable with sufficient patience.
posted by JackFlash at 10:25 PM on October 17, 2012

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