<p>I took a quick look at the 2021 <a href="https://arxiv.org/abs/2009.08987" rel="nofollow noreferrer">arXiv preprint</a>, and it does feel a bit &quot;rushed&quot;.</p>
<p>I see the pair (45 AU, 20M☉) in many equations, and it seems to say (circa Eq. 5) that if the diameter is roughly that of Saturn, the corresponding orbital velocity using (45 AU, 20M☉) matches the duration of the occultation, which is the only real, direct, quantitative measurement <em>of the proposed planet</em> that they have.</p>
<p>And <span class="math-container">$$\sqrt{\frac{45^3}{20}} = 67.5 $$</span> does yield almost 70 years.</p>
<p>The building up an argument for an orbit (size, period) from just a single occultation duration and a lot of secondary assumptions for this object was already asked about and answered at my 2021 question about the same arXiv preprint.</p>
<p>But note, <em>this is not a duplicate question!</em> I only asked about the method, not the numbers themselves.</p>
<ul>
<li><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></li>
</ul>
<p>Ignoring the first two popular news items and looking only at the Oct 25, 2021 NASA News item <a href="https://science.nasa.gov/universe/exoplanets/exoplanet-discoveries/first-evidence-of-a-planet-identified-beyond-our-galaxy/" rel="nofollow noreferrer">First Evidence of a Planet Identified Beyond Our Galaxy</a> I find the following blurbs:</p>
<p>The first of which stimulated my linked question:</p>
<blockquote>
<p>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.</p>
</blockquote>
<p>The second seems to have two possible &quot;errors&quot; or misquotes.</p>
<blockquote>
<p>This binary system contains a black hole or neutron star orbiting a companion star with a mass about 20 times that of the Sun. The X-ray transit they found using Chandra data lasted about three hours, during which the X-ray emission decreased to zero. Based on this and other information, the researchers estimate the exoplanet candidate in M51-ULS-1 would be roughly the size of Saturn, <strong>and orbit the neutron star or black hole</strong> at about <strong>twice the distance of Saturn from the Sun.</strong></p>
</blockquote>
<ol>
<li>If I understand the preprint correctly, the planet orbits BOTH the &quot;the neutron star or black hole&quot; and its much more massive &quot;companion star with a mass about 20 times that of the Sun.&quot; It should say it orbits the massive <em>donor start</em> and its less massive but X-ray generating <em>acceptor companion</em>, which is either a neutron star or black hole.</li>
<li>&quot;...twice the distance of Saturn from the Sun.&quot; seems to me to be inconsistent with the preprint. So far it looks like they settled on the pair (45 AU, 20M☉). <strong>I think that this was written based on interviews, as a rushed press release, as it contains several quotes from various people but no link to the preprint.</strong></li>
</ol>
<h3>Conclusion:</h3>
<p>We should not take popular news items as sources of facts. In fact, we should not necessarily assume that an arXiv preprint is the final word on a result!</p>
<p><strong>At least look for the <a href="https://www.nature.com/articles/s41550-021-01495-w" rel="nofollow noreferrer">peer-reviewed version that's been published</a>, any comments posted in the journal on it, and any subsequent papers written about this object, or <a href="https://scholar.google.com/scholar?cites=15953667715528441935&amp;as_sdt=2005&amp;sciodt=0,5&amp;hl=en" rel="nofollow noreferrer">articles that cite it</a>.</strong></p>

CC BY-SA 4.0
2024-08-20T04:49:52.117Z
Thanks! So would the combined mass of the donor star and compact object be 20 solar masses or just the donor star? Also would a bh or neutron star be more likely in this situation? And do you have any estimation for the black hole mass (if it is one) other than "around 10 solar masses"?
ArchiveOfStars
CC BY-SA 4.0
2024-08-20T04:56:08.600Z
@ArchiveOfStars *They simply don't know, and try to avoid saying with any certainty!* It's all plausible deductive guessing.
uhoh
CC BY-SA 4.0
2024-08-20T00:39:11.287Z
2024-08-20T01:07:13.680Z
2024-08-20T01:07:13.680Z
uhoh
<p>I've been doing research into the exoplanet candidate of M51-ULS-1b, an exoplanet thought to orbit a high mass x-ray binary in the whirlpool galaxy. However I had noticed something odd when adding up the propused orbital period and semi-major axis.</p>
<p><a href="https://www.astronomy.com/science/found-the-first-exoplanet-outside-of-our-milky-way/" rel="nofollow noreferrer">Astronomy Magazine</a>, <a href="https://www.space.com/first-planet-outside-milky-way-discovery" rel="nofollow noreferrer">Space. com</a> and even <a href="https://science.nasa.gov/universe/exoplanets/exoplanet-discoveries/first-evidence-of-a-planet-identified-beyond-our-galaxy/" rel="nofollow noreferrer">NASA. gov</a> all list the exoplanet to have an orbital period of around 70 years and a semi major axis of around 19.2 au. It also mentions that the companion star is around 20 solar masses and the <a href="https://arxiv.org/abs/2009.08987" rel="nofollow noreferrer">original paper</a> states that the compact object (assuming neutron star) is either 1.4 solar masses or (assuming black hole) around 10 solar masses. Even assuming the lowest masses for the star and companion, for it to have an orbital period of 70 yrs it would need a semi major axis of 47 au, and if it had a semi major axis of 19.2 au it's orbital period would be 18 years. The original paper seems to have a more realistic semi major axis of 45 au that it briefly mentioned, however it still would fall short assuming a companion black hole (semi major axis of 52 au for 70 year period). Also if we take into accound the observed transit velocity is 17 km/s, than assuming even the lowest mass estimations the semi major axis should be 60 au and the orbital period would be 100 years, and assuming black hole mass estimations the semi major axis would be 90 au and the orbital period would be 150 years! The only thing that i can think is if the orbital period was 70 yrs and the semi major axis 45 au, but then the combined mass of both compact object and star would be around 18 solar masses and the orbtial velocity would be 19 km/s. So who's wrong here, and which number did they mess up? And who's wrong? How would a saturn mass exoplanet completeit block x ray emissions from 50 au? Could the exoplanet be &quot;puffed up&quot; like seen in many hot jupiters? Or is the orbital period wrong and does it tightly orbit the binary? Also how/why/where did all these sources mess up? And did they really mess up this badly or am I doing my calculations wrong? Someone please explain.</p>

CC BY-SA 4.0
2024-08-20T00:52:02.807Z
[Do we really want a "debunk" tag?](https://astronomy.meta.stackexchange.com/q/972/7982) In general we should avoid questions about 3 year old popular news summaries of rapidly evolving events like new discoveries, and do a bit of due diligence. Scientifically inaccurate sentences are added to the internet at kHz if not MHz, and so are not really good sources for the basis of Stack Exchange questiosn by themselves.
uhoh
CC BY-SA 4.0
2024-08-20T07:19:08.420Z
The  NASA press release is wrong, the others just copied it. The X-ray source region is much smaller than Saturn, so a Saturn-sized object can block it.
ProfRob
<exoplanet><orbital-mechanics><binary-star>
Orbital period and semi major axis of M51-ULS-1b dont add up
ArchiveOfStars
# Orbital period and semi major axis of M51-ULS-1b dont add up

(Asked by: username_0 on 2024-08-19)

I've been doing research into the exoplanet candidate of M51-ULS-1b, an exoplanet thought to orbit a high mass x-ray binary in the whirlpool galaxy. However I had noticed something odd when adding up the propused orbital period and semi-major axis.

[Astronomy Magazine](https://www.astronomy.com/science/found-the-first-exoplanet-outside-of-our-milky-way/), [Space. com](https://www.space.com/first-planet-outside-milky-way-discovery) and even [NASA. gov](https://science.nasa.gov/universe/exoplanets/exoplanet-discoveries/first-evidence-of-a-planet-identified-beyond-our-galaxy/) all list the exoplanet to have an orbital period of around 70 years and a semi major axis of around 19.2 au. It also mentions that the companion star is around 20 solar masses and the [original paper](https://arxiv.org/abs/2009.08987) states that the compact object (assuming neutron star) is either 1.4 solar masses or (assuming black hole) around 10 solar masses. Even assuming the lowest masses for the star and companion, for it to have an orbital period of 70 yrs it would need a semi major axis of 47 au, and if it had a semi major axis of 19.2 au it's orbital period would be 18 years. The original paper seems to have a more realistic semi major axis of 45 au that it briefly mentioned, however it still would fall short assuming a companion black hole (semi major axis of 52 au for 70 year period). Also if we take into accound the observed transit velocity is 17 km/s, than assuming even the lowest mass estimations the semi major axis should be 60 au and the orbital period would be 100 years, and assuming black hole mass estimations the semi major axis would be 90 au and the orbital period would be 150 years! The only thing that i can think is if the orbital period was 70 yrs and the semi major axis 45 au, but then the combined mass of both compact object and star would be around 18 solar masses and the orbtial velocity would be 19 km/s. So who's wrong here, and which number did they mess up? And who's wrong? How would a saturn mass exoplanet completeit block x ray emissions from 50 au? Could the exoplanet be "puffed up" like seen in many hot jupiters? Or is the orbital period wrong and does it tightly orbit the binary? Also how/why/where did all these sources mess up? And did they really mess up this badly or am I doing my calculations wrong? Someone please explain.

**Tags:** exoplanet, orbital-mechanics, binary-star

> **Comments:**
> **username_1**: [Do we really want a "debunk" tag?](https://astronomy.meta.stackexchange.com/q/972/7982) In general we should avoid questions about 3 year old popular news summaries of rapidly evolving events like new discoveries, and do a bit of due diligence. Scientifically inaccurate sentences are added to the internet at kHz if not MHz, and so are not really good sources for the basis of Stack Exchange questiosn by themselves.  
> **username_2**: The  NASA press release is wrong, the others just copied it. The X-ray source region is much smaller than Saturn, so a Saturn-sized object can block it.  

---

## 1 answers

### Answer by username_1 on 2024-08-20. Score: 2 | Accepted answer

I took a quick look at the 2021 [arXiv preprint](https://arxiv.org/abs/2009.08987), and it does feel a bit "rushed".

I see the pair (45 AU, 20M☉) in many equations, and it seems to say (circa Eq. 5) that if the diameter is roughly that of Saturn, the corresponding orbital velocity using (45 AU, 20M☉) matches the duration of the occultation, which is the only real, direct, quantitative measurement *of the proposed planet* that they have.

And $$\sqrt{\frac{45^3}{20}} = 67.5 $$ does yield almost 70 years.

The building up an argument for an orbit (size, period) from just a single occultation duration and a lot of secondary assumptions for this object was already asked about and answered at my 2021 question about the same arXiv preprint.

But note, *this is not a duplicate question!* I only asked about the method, not the numbers themselves.

* [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)

Ignoring the first two popular news items and looking only at the Oct 25, 2021 NASA News item [First Evidence of a Planet Identified Beyond Our Galaxy](https://science.nasa.gov/universe/exoplanets/exoplanet-discoveries/first-evidence-of-a-planet-identified-beyond-our-galaxy/) I find the following blurbs:

The first of which stimulated my linked question:

> 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.

The second seems to have two possible "errors" or misquotes.

> This binary system contains a black hole or neutron star orbiting a companion star with a mass about 20 times that of the Sun. The X-ray transit they found using Chandra data lasted about three hours, during which the X-ray emission decreased to zero. Based on this and other information, the researchers estimate the exoplanet candidate in M51-ULS-1 would be roughly the size of Saturn, **and orbit the neutron star or black hole** at about **twice the distance of Saturn from the Sun.**

1. If I understand the preprint correctly, the planet orbits BOTH the "the neutron star or black hole" and its much more massive "companion star with a mass about 20 times that of the Sun." It should say it orbits the massive *donor start* and its less massive but X-ray generating *acceptor companion*, which is either a neutron star or black hole.
2. "...twice the distance of Saturn from the Sun." seems to me to be inconsistent with the preprint. So far it looks like they settled on the pair (45 AU, 20M☉). **I think that this was written based on interviews, as a rushed press release, as it contains several quotes from various people but no link to the preprint.**

### Conclusion:

We should not take popular news items as sources of facts. In fact, we should not necessarily assume that an arXiv preprint is the final word on a result!

**At least look for the [peer-reviewed version that's been published](https://www.nature.com/articles/s41550-021-01495-w), any comments posted in the journal on it, and any subsequent papers written about this object, or [articles that cite it](https://scholar.google.com/scholar?cites=15953667715528441935&as_sdt=2005&sciodt=0,5&hl=en).**

> **Comments:**
> **username_0**: Thanks! So would the combined mass of the donor star and compact object be 20 solar masses or just the donor star? Also would a bh or neutron star be more likely in this situation? And do you have any estimation for the black hole mass (if it is one) other than "around 10 solar masses"?  
> **username_1**: @username_0 *They simply don't know, and try to avoid saying with any certainty!* It's all plausible deductive guessing.  

---

