<p>The distance moduli at 5, 500, 50,000 and 5,000,000 are -4.1, 5.9, 15.9 and 25.9 respectively. We add that to the absolute visual magnitude of the Sun, 4.8, to get apparent magnitudes of 0.7, 10.7, 20.7 and 30.7 respectively.</p>

<p>Most planets are detected by the Doppler wobble in the parent star or if the planet transits in front of the parent star. Some are detected by direct imaging, but these are planets much larger than Jupiter, orbiting at much greater distances than Jupiter (although see below).</p>

<p>Both the main detection techniques require relatively bright stars. The Doppler wobble technique relies on measuring small velocity shifts. These would be of the order of 10 m/s caused by Jupiter, but only 7 cm/s caused by the Earth. To claim a detection requires observing the star over two orbits (at least). Current technology would easily detect Jupiter, probably Saturn, but possibly not the Earth, if you observed for long enough. But to collect enough photons requires a star brighter than about 13-15 mag. So only your closest distances are possible.</p>

<p>The transiting technique has found (large) planets around stars a little fainter than this, but these tend to be in short period orbits. No Jupiter-like (in terms of size and orbit) have been founds by transits and no Earth-like ones either. You could just detect a Venus-like planet for your closest two distances. There is also the factor of orientation to consider. The probability of alignment such that a transit occurs is something like stellar radius/planet orbit radius. For Venus this about <span class="math-container">$6\times 10^{-3}$</span>, and even more unlikely for the more distant planets.</p>

<p>So my conclusion is yes, you would detect planets at the first two distances using the Doppler wobble technique (providing you observed for 20 years or so). But you wouldn't see anything by looking for transits, unless you were very lucky and spotted Venus, again at your closest two distances.</p>

<p>More information and detail
.
<a href="https://astronomy.stackexchange.com/questions/8605/if-alpha-centauri-as-solar-system-exactly-mirrored-our-own-what-would-we-be-ab/8628?r=SearchResults#8628">If Alpha Centauri A&#39;s solar system exactly mirrored our own, what would we be able to detect?</a></p>

<p><strong>Note added.</strong></p>

<p>There are currently instruments probing Alpha Centauri for planets, by direct imaging. This corresponds to your first distance. I need to check their sensitivities...</p>

<p>There is also a possibility <em>in the near future</em> (with SKA) of detecting radio emissions from an Earth-like planet/civilisation at the first of your distances. See <a href="https://astronomy.stackexchange.com/a/10638/2531">https://astronomy.stackexchange.com/a/10638/2531</a></p>

CC BY-SA 4.0
2020-03-18T06:23:25.410Z
Isn't the probability of alignment instead proportional to star radius/planet orbit radius, so $10^{-3}$ for Jupiter and $6\times10^{-3}$ for Venus? Planet radius affects the amount of light blocked but not the probability.
nanoman
CC BY-SA 4.0
2020-03-18T07:37:49.910Z
@nanoman totally correct! (There is only a second order effect due to planet radius)
ProfRob
CC BY-SA 4.0
2020-03-26T03:19:17.043Z
@ Rob Jeffries Are you sure it would take 20 years to detect Jupiter? There are numerous planets in the Exoplanet catalogues with long periods based on partial orbits. I suppose there are fairly high uncertainties in the periods, though.
Jack R. Woods
CC BY-SA 4.0
2020-03-26T08:04:54.680Z
@JackR.Woods 2 periods is probably playing it safe. Call it one period if you like, it doesn't change the answer.
ProfRob
CC BY-SA 4.0
2020-03-27T21:39:55.303Z
It was very hard to choose an answer to accept. Thank you very much, in either case!
Ink blot
CC BY-SA 4.0
2020-03-17T07:17:59.193Z
2020-03-18T07:42:06.553Z
2020-03-18T07:42:06.553Z
ProfRob
<p>An (almost-)omnipotent being selects the solar system, presses Ctrl+C, and then Ctrl+V several times, creating copies at distances of 5, 500, 50.000, and 5.000.000 light-years away. All in a direction where the instruments we have are currently directed that way.</p>

<p>So we see these four star systems at various distances. </p>

<p>With our current technology, of 2020, which of the planets will we discover in each of these distances? </p>

CC BY-SA 4.0
2020-03-16T22:58:15.717Z
Are there any other relevant tags? I couldn't think of any.
Ink blot
CC BY-SA 4.0
2020-03-16T23:42:50.657Z
5 million light years away ?  Wouldn't even find the star, never mind the planets.
StephenG - Help Ukraine
CC BY-SA 4.0
2020-03-16T23:59:08.447Z
The exoplanet 'population plot' aka the period-mass diagram with the solar system planets is fairly ubiquitious in talks, and should be easily googlable.
AtmosphericPrisonEscape
CC BY-SA 4.0
2020-03-17T21:29:19.547Z
Does this answer your question? [Is it odd that our Sun has so many planets?](https://astronomy.stackexchange.com/questions/13301/is-it-odd-that-our-sun-has-so-many-planets)
Mark
CC BY-SA 4.0
2020-03-17T21:32:57.267Z
@Mark: I don't think it's quite the same. I'm not asking about other star systems (although it did motivate the question in some sense), but rather about our technology for making these discoveries.
Ink blot
CC BY-SA 4.0
2020-03-17T23:58:27.167Z
@Mark I think the two answers *posted here* 16 hours ago answer this question *much better* than answers there. I don't think closing this question as duplicate of that is the right thing to do here. Have you read the two answers here and found them inferior answers to *this question* compared to those in some way? We close as duplicate based on answers, not similarity of questions.
uhoh
CC BY-SA 4.0
2020-03-18T00:43:46.993Z
@uhoh “We close as duplicate based on answers, not similarity of questions” isn’t quite right, it’s more complex than that. You’re right that the relevance of *answers* is one key factor in deciding if a question is a duplicate, but you can close a question as dupe of another even if the original doesn’t have an answer.
Chappo Hasn't Forgotten
CC BY-SA 4.0
2020-03-18T00:55:43.220Z
@ChappoHasn'tForgottenMonica I know moderators can do that, but can users in general really do that? Also using terms like "original" suggest a bias towards closing a newer question as duplicate to an older question, but duping decisions should be chronology-agnostic.
uhoh
CC BY-SA 4.0
2020-03-19T02:17:59.603Z
@uhoh in the VTC box you can choose "duplicate" and paste the address of any question, including one without answers & even one closed as a dupe itself. Yes, I agree on chronology-agnostic: what we're trying to achieve is the best user experience, i.e. *what gets the user to the most relevant answer as directly as possible*. For example, a more recent question might have been closed as a dupe of an older question, but the older one's answers might be lousy and the newer one might have a great answer. Ideally, such dupes should be *merged* but I think that needs diamond powers. :-)
Chappo Hasn't Forgotten
CC BY-SA 4.0
2020-03-19T02:25:35.457Z
@ChappoHasn'tForgottenMonica Thanks! I didn't know that the VTC box would accept an unanswered question, interesting...
uhoh
<observational-astronomy><exoplanet>
Which of the planets would be detected if they were exoplanets?
Ink blot
# Which of the planets would be detected if they were exoplanets?

(Asked by: username_0 on 2020-03-16)

An (almost-)omnipotent being selects the solar system, presses Ctrl+C, and then Ctrl+V several times, creating copies at distances of 5, 500, 50.000, and 5.000.000 light-years away. All in a direction where the instruments we have are currently directed that way.

So we see these four star systems at various distances.

With our current technology, of 2020, which of the planets will we discover in each of these distances?

**Tags:** observational-astronomy, exoplanet

> **Comments:**
> **username_0**: Are there any other relevant tags? I couldn't think of any.  
> **username_1**: 5 million light years away ?  Wouldn't even find the star, never mind the planets.  
> **username_2**: The exoplanet 'population plot' aka the period-mass diagram with the solar system planets is fairly ubiquitious in talks, and should be easily googlable.  
> **username_3**: Does this answer your question? [Is it odd that our Sun has so many planets?](https://astronomy.stackexchange.com/questions/13301/is-it-odd-that-our-sun-has-so-many-planets)  
> **username_0**: @username_3: I don't think it's quite the same. I'm not asking about other star systems (although it did motivate the question in some sense), but rather about our technology for making these discoveries.  
> **username_4**: @username_3 I think the two answers *posted here* 16 hours ago answer this question *much better* than answers there. I don't think closing this question as duplicate of that is the right thing to do here. Have you read the two answers here and found them inferior answers to *this question* compared to those in some way? We close as duplicate based on answers, not similarity of questions.  
> **username_5**: @username_4 “We close as duplicate based on answers, not similarity of questions” isn’t quite right, it’s more complex than that. You’re right that the relevance of *answers* is one key factor in deciding if a question is a duplicate, but you can close a question as dupe of another even if the original doesn’t have an answer.  
> **username_4**: @ChappoHasn'tForgottenMonica I know moderators can do that, but can users in general really do that? Also using terms like "original" suggest a bias towards closing a newer question as duplicate to an older question, but duping decisions should be chronology-agnostic.  
> **username_5**: @username_4 in the VTC box you can choose "duplicate" and paste the address of any question, including one without answers & even one closed as a dupe itself. Yes, I agree on chronology-agnostic: what we're trying to achieve is the best user experience, i.e. *what gets the user to the most relevant answer as directly as possible*. For example, a more recent question might have been closed as a dupe of an older question, but the older one's answers might be lousy and the newer one might have a great answer. Ideally, such dupes should be *merged* but I think that needs diamond powers. :-)  
> **username_4**: @ChappoHasn'tForgottenMonica Thanks! I didn't know that the VTC box would accept an unanswered question, interesting...  

---

## 1 answers

### Answer by username_6 on 2020-03-17. Score: 23

The distance moduli at 5, 500, 50,000 and 5,000,000 are -4.1, 5.9, 15.9 and 25.9 respectively. We add that to the absolute visual magnitude of the Sun, 4.8, to get apparent magnitudes of 0.7, 10.7, 20.7 and 30.7 respectively.

Most planets are detected by the Doppler wobble in the parent star or if the planet transits in front of the parent star. Some are detected by direct imaging, but these are planets much larger than Jupiter, orbiting at much greater distances than Jupiter (although see below).

Both the main detection techniques require relatively bright stars. The Doppler wobble technique relies on measuring small velocity shifts. These would be of the order of 10 m/s caused by Jupiter, but only 7 cm/s caused by the Earth. To claim a detection requires observing the star over two orbits (at least). Current technology would easily detect Jupiter, probably Saturn, but possibly not the Earth, if you observed for long enough. But to collect enough photons requires a star brighter than about 13-15 mag. So only your closest distances are possible.

The transiting technique has found (large) planets around stars a little fainter than this, but these tend to be in short period orbits. No Jupiter-like (in terms of size and orbit) have been founds by transits and no Earth-like ones either. You could just detect a Venus-like planet for your closest two distances. There is also the factor of orientation to consider. The probability of alignment such that a transit occurs is something like stellar radius/planet orbit radius. For Venus this about $6\times 10^{-3}$, and even more unlikely for the more distant planets.

So my conclusion is yes, you would detect planets at the first two distances using the Doppler wobble technique (providing you observed for 20 years or so). But you wouldn't see anything by looking for transits, unless you were very lucky and spotted Venus, again at your closest two distances.

More information and detail
.
[If Alpha Centauri A's solar system exactly mirrored our own, what would we be able to detect?](https://astronomy.stackexchange.com/questions/8605/if-alpha-centauri-as-solar-system-exactly-mirrored-our-own-what-would-we-be-ab/8628?r=SearchResults#8628)

**Note added.**

There are currently instruments probing Alpha Centauri for planets, by direct imaging. This corresponds to your first distance. I need to check their sensitivities...

There is also a possibility *in the near future* (with SKA) of detecting radio emissions from an Earth-like planet/civilisation at the first of your distances. See <https://astronomy.stackexchange.com/a/10638/2531>

> **Comments:**
> **username_7**: Isn't the probability of alignment instead proportional to star radius/planet orbit radius, so $10^{-3}$ for Jupiter and $6\times10^{-3}$ for Venus? Planet radius affects the amount of light blocked but not the probability.  
> **username_6**: @username_7 totally correct! (There is only a second order effect due to planet radius)  
> **username_8**: @ Rob Jeffries Are you sure it would take 20 years to detect Jupiter? There are numerous planets in the Exoplanet catalogues with long periods based on partial orbits. I suppose there are fairly high uncertainties in the periods, though.  
> **username_6**: @JackR.Woods 2 periods is probably playing it safe. Call it one period if you like, it doesn't change the answer.  
> **username_0**: It was very hard to choose an answer to accept. Thank you very much, in either case!  

---

