<blockquote>
<p>Is one transit enough to be confident that we detected an exoplanet?</p>
</blockquote>
<p>I will argue:</p>
<h3>No.</h3>
<p>and back it up with material contained in my earlier question <a href="https://astronomy.stackexchange.com/q/47224/7982">After only one eclipse of its X-ray bright primary, how can astronomers estimate the first extragalactic exoplanet&#39;s period to be about 70 years?</a></p>
<blockquote>
<p>Phys.org's <a href="https://phys.org/news/2021-10-astronomers-planet-galaxy.html" rel="noreferrer">Astronomers may have discovered the first planet outside of our galaxy</a> links to Di Stefano et al. 2021 <a href="https://www.nature.com/articles/s41550-021-01495-w" rel="noreferrer">A <em><strong>possible</strong></em> planet candidate in an external galaxy detected through X-ray transit</a> (earlier <a href="https://arxiv.org/abs/2009.08987" rel="noreferrer">arXiv preprint</a>) and says:</p>
<blockquote>
<p><strong>While this is a tantalizing study, more data would be needed to verify the interpretation as an extragalactic exoplanet.</strong> One challenge is that the planet candidate's large orbit means it would not cross in front of its binary partner again for about 70 years, <strong>thwarting any attempts for a confirming observation for decades.</strong></p>
<p>&quot;Unfortunately to confirm that we're seeing a planet we would likely have to wait decades to see another transit,&quot; said co-author Nia Imara of the University of California at Santa Cruz. &quot;And because of the uncertainties about how long it takes to orbit, we wouldn't know exactly when to look.&quot;</p>
<p>Can the dimming have been caused by a cloud of gas and dust passing in front of the X-ray source? The researchers consider this to be an unlikely explanation, as the characteristics of the event observed in M51-ULS-1 are not consistent with the passage of such a cloud. The model of a planet candidate is, however, consistent with the data.</p>
</blockquote>
</blockquote>
<p>See the excellent answers there to get a better idea of the great extent to which the authors have build a scientific case for &quot;exoplanetship&quot; without actually calling it an exoplanet.</p>
<p>Unless otherwise stated, orbits of planets (exos or in our own system) are pretty much <em>periodic</em> and I think that closed, well-defined and roughly periodic orbits are part of the definition of what a planet is, even if it's not explicitly stated as such.</p>
<p>How do we know if a transit is from an orbiting body?</p>
<p>As you point out transit <em>periodicity</em> is a hallmark of an exoplanet identification using the transit method (as it is for spectroscopic methods) and without a few repeats you don't have periodicity.</p>
<p>Accretion material, starspots, and unkown/unidentified intervening bodies are just some of the ways that a single dip might happen.</p>
<p>So even with all the circumstantial evidence cited within Di Stefano et al. 2021, the authors are careful to make clear the difference between a dip <em>consistent with</em> that of an exoplanet, and a likely exoplanet or confirmed exoplanet.</p>

CC BY-SA 4.0
2022-03-06T04:24:32.690Z
2022-03-06T04:24:32.690Z
uhoh
<p>Everything is pretty much in the title. I was thinking about the transit method and was wondering if the signature of a transit on the light curve of a star is distinctive enough to only need one transit for confirmation. I know that to deduce other information like the period or the distance of the planet you need more transit, but is one enough to confirm the presence of an exoplanet ?</p>
<p>Is there other phenomena that can produce this type of light curve ? For example if a cloud of gas would pass in front of the star i would expect the brightness to drop for a long time. I would expect also the probability of a small solid objects passing in front of the star far away such that it's angular size would be the same as of a planet would be very rare.</p>
<p>Or maybe is it because our measurement is too noisy and we need to correlate a signal over time to be confident that we observed a transit ?</p>

CC BY-SA 4.0
2022-03-05T15:42:21.063Z
There are other false-positive signals that produce transits - e.g. unresolved background eclipsing binaries, grazing incidence eclipses and others.
ProfRob
CC BY-SA 4.0
2022-03-05T21:40:11.583Z
related: [After only one eclipse of its X-ray bright primary, how can astronomers estimate the first extragalactic exoplanet's period to be about 70 years?](https://astronomy.stackexchange.com/q/47224/7982)
uhoh
CC BY-SA 4.0
2022-03-06T05:31:41.207Z
[Kepler and the Transiting Exoplanet Survey Satellite (TESS) requires at least two transits -- that is, two equal dips in brightness from the same star to detect exoplanets.](https://exoplanets.nasa.gov/news/1524/how-do-you-find-and-confirm-a-planet-10-things-about-the-search-for-exoplanets/)
Nilay Ghosh
<observational-astronomy><exoplanet><planetary-transits>
Is one transit enough to be confident that we detected an exoplanet
Tackwin
# Is one transit enough to be confident that we detected an exoplanet

(Asked by: username_0 on 2022-03-05)

Everything is pretty much in the title. I was thinking about the transit method and was wondering if the signature of a transit on the light curve of a star is distinctive enough to only need one transit for confirmation. I know that to deduce other information like the period or the distance of the planet you need more transit, but is one enough to confirm the presence of an exoplanet ?

Is there other phenomena that can produce this type of light curve ? For example if a cloud of gas would pass in front of the star i would expect the brightness to drop for a long time. I would expect also the probability of a small solid objects passing in front of the star far away such that it's angular size would be the same as of a planet would be very rare.

Or maybe is it because our measurement is too noisy and we need to correlate a signal over time to be confident that we observed a transit ?

**Tags:** observational-astronomy, exoplanet, planetary-transits

> **Comments:**
> **username_1**: There are other false-positive signals that produce transits - e.g. unresolved background eclipsing binaries, grazing incidence eclipses and others.  
> **username_2**: related: [After only one eclipse of its X-ray bright primary, how can astronomers estimate the first extragalactic exoplanet's period to be about 70 years?](https://astronomy.stackexchange.com/q/47224/7982)  
> **username_3**: [Kepler and the Transiting Exoplanet Survey Satellite (TESS) requires at least two transits -- that is, two equal dips in brightness from the same star to detect exoplanets.](https://exoplanets.nasa.gov/news/1524/how-do-you-find-and-confirm-a-planet-10-things-about-the-search-for-exoplanets/)  

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## 1 answers

### Answer by username_2 on 2022-03-06. Score: 8 | Accepted answer

> Is one transit enough to be confident that we detected an exoplanet?

I will argue:

### No.

and back it up with material contained in my earlier question [After only one eclipse of its X-ray bright primary, how can astronomers estimate the first extragalactic exoplanet's period to be about 70 years?](https://astronomy.stackexchange.com/q/47224/7982)

> Phys.org's [Astronomers may have discovered the first planet outside of our galaxy](https://phys.org/news/2021-10-astronomers-planet-galaxy.html) links to Di Stefano et al. 2021 [A ***possible*** planet candidate in an external galaxy detected through X-ray transit](https://www.nature.com/articles/s41550-021-01495-w) (earlier [arXiv preprint](https://arxiv.org/abs/2009.08987)) and says:
>
> > **While this is a tantalizing study, more data would be needed to verify the interpretation as an extragalactic exoplanet.** One challenge is that the planet candidate's large orbit means it would not cross in front of its binary partner again for about 70 years, **thwarting any attempts for a confirming observation for decades.**
> >
> > "Unfortunately to confirm that we're seeing a planet we would likely have to wait decades to see another transit," said co-author Nia Imara of the University of California at Santa Cruz. "And because of the uncertainties about how long it takes to orbit, we wouldn't know exactly when to look."
> >
> > Can the dimming have been caused by a cloud of gas and dust passing in front of the X-ray source? The researchers consider this to be an unlikely explanation, as the characteristics of the event observed in M51-ULS-1 are not consistent with the passage of such a cloud. The model of a planet candidate is, however, consistent with the data.

See the excellent answers there to get a better idea of the great extent to which the authors have build a scientific case for "exoplanetship" without actually calling it an exoplanet.

Unless otherwise stated, orbits of planets (exos or in our own system) are pretty much *periodic* and I think that closed, well-defined and roughly periodic orbits are part of the definition of what a planet is, even if it's not explicitly stated as such.

How do we know if a transit is from an orbiting body?

As you point out transit *periodicity* is a hallmark of an exoplanet identification using the transit method (as it is for spectroscopic methods) and without a few repeats you don't have periodicity.

Accretion material, starspots, and unkown/unidentified intervening bodies are just some of the ways that a single dip might happen.

So even with all the circumstantial evidence cited within Di Stefano et al. 2021, the authors are careful to make clear the difference between a dip *consistent with* that of an exoplanet, and a likely exoplanet or confirmed exoplanet.


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