<p>A frequently cited study about <a href="https://en.wikipedia.org/wiki/Planet_Nine" rel="nofollow noreferrer">Planet Nine</a> is <a href="https://arxiv.org/abs/1601.05438" rel="nofollow noreferrer">arXiv:1601.05438</a>: <em>Evidence for a Distant Giant Planet in the Solar System</em>, but I can confirm that after 2016 there are not too many studies on the subject, at least I could not find many. There are two interesting studies I would like to share, but none of them spoke of <em>four</em> candidates.</p>
<ol>
<li><p><a href="https://iopscience.iop.org/article/10.3847/1538-4357/aba633/meta" rel="nofollow noreferrer">Spitzer Follow-up of Extremely Cold Brown Dwarfs Discovered by the Backyard Worlds: Planet 9 Citizen Science Project</a>. The abstracts reads</p>
<blockquote>
<p>We present Spitzer follow-up imaging of 95 candidate extremely cold brown dwarfs discovered by the Backyard Worlds: Planet 9 citizen science project, which uses visually perceived motion in multiepoch Wide-field Infrared Survey Explorer (WISE) images to identify previously unrecognized substellar neighbors to the Sun. [...]</p>
</blockquote>
</li>
<li><p>Even more interesting is <a href="https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.125.051103" rel="nofollow noreferrer">What If Planet 9 Is a Primordial Black Hole?</a> by Jakub Scholtz and James Unwin from 2020:</p>
<blockquote>
<p>We highlight that the anomalous orbits of trans-Neptunian objects (TNOs) and an excess in microlensing events in the 5-year Optical Gravitational Lensing Experiment data set can be simultaneously explained by a new population of astrophysical bodies with mass several times that of the Earth (M⊕). We take these objects to be primordial black holes (PBHs) and point out the orbits of TNOs would be altered if one of these PBHs was captured by the Solar System, inline with the Planet 9 hypothesis. Capture of a free floating planet is a leading explanation for the origin of Planet 9, and we show that the probability of capturing a PBH instead is comparable. The observational constraints on a PBH in the outer Solar System significantly differ from the case of a new ninth planet. This scenario could be confirmed through annihilation signals from the dark matter microhalo around the PBH.</p>
</blockquote>
</li>
</ol>
<p>I hope that this is helpful for you.</p>

CC BY-SA 4.0
2021-03-03T13:15:16.713Z
2021-03-03T14:16:58.107Z
2021-03-03T14:16:58.107Z
B--rian
<p>From <a href="http://en.wikipedia.org/wiki/Planet_Nine#Citizen_science" rel="noreferrer">Wikipedia Planet Nine#Citizen science</a>:</p>

<blockquote>
  <p>In April 2017, using data from the SkyMapper telescope at Siding Spring Observatory, citizen scientists on the Zooniverse platform reported four candidates for Planet Nine. These candidates will be followed up on by astronomers to determine their viability. The project, which started on 28 March 2017, completed their goals in less than three days with around five million classifications by more than 60,000 individuals.</p>
</blockquote>

<p>But it has not been updated for nearly 3 years. Is there any follow up study that confirm what these 4 candidates actually are?</p>

CC BY-SA 4.0
2021-02-09T21:08:53.570Z
Here's the [2017 ANU press release](https://www.anu.edu.au/news/all-news/four-unknown-objects-being-investigated-in-planet-9-search). I'm not finding any research publications.
Mike G
CC BY-SA 4.0
2021-03-03T14:21:05.413Z
I also found https://www.zooniverse.org/projects/skymap/planet-9/about/results which just says "Stay tuned to see our exciting results!". That seems the (follow-up?) project quoted in Wikipedia, isn't it?
B--rian
<planet><9th-planet>
What are the 4 candidates for Planet Nine reported in 2017?
NeutronCat
# What are the 4 candidates for Planet Nine reported in 2017?

(Asked by: username_0 on 2020-02-26)

From [Wikipedia Planet Nine#Citizen science](http://en.wikipedia.org/wiki/Planet_Nine#Citizen_science):

> In April 2017, using data from the SkyMapper telescope at Siding Spring Observatory, citizen scientists on the Zooniverse platform reported four candidates for Planet Nine. These candidates will be followed up on by astronomers to determine their viability. The project, which started on 28 March 2017, completed their goals in less than three days with around five million classifications by more than 60,000 individuals.

But it has not been updated for nearly 3 years. Is there any follow up study that confirm what these 4 candidates actually are?

**Tags:** planet, 9th-planet

> **Comments:**
> **username_1**: Here's the [2017 ANU press release](https://www.anu.edu.au/news/all-news/four-unknown-objects-being-investigated-in-planet-9-search). I'm not finding any research publications.  
> **username_2**: I also found https://www.zooniverse.org/projects/skymap/planet-9/about/results which just says "Stay tuned to see our exciting results!". That seems the (follow-up?) project quoted in Wikipedia, isn't it?  

---

## 1 answers

### Answer by username_2 on 2021-03-03. Score: 2

A frequently cited study about [Planet Nine](https://en.wikipedia.org/wiki/Planet_Nine) is [arXiv:1601.05438](https://arxiv.org/abs/1601.05438): *Evidence for a Distant Giant Planet in the Solar System*, but I can confirm that after 2016 there are not too many studies on the subject, at least I could not find many. There are two interesting studies I would like to share, but none of them spoke of *four* candidates.

1. [Spitzer Follow-up of Extremely Cold Brown Dwarfs Discovered by the Backyard Worlds: Planet 9 Citizen Science Project](https://iopscience.iop.org/article/10.3847/1538-4357/aba633/meta). The abstracts reads

   > We present Spitzer follow-up imaging of 95 candidate extremely cold brown dwarfs discovered by the Backyard Worlds: Planet 9 citizen science project, which uses visually perceived motion in multiepoch Wide-field Infrared Survey Explorer (WISE) images to identify previously unrecognized substellar neighbors to the Sun. [...]
2. Even more interesting is [What If Planet 9 Is a Primordial Black Hole?](https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.125.051103) by Jakub Scholtz and James Unwin from 2020:

   > We highlight that the anomalous orbits of trans-Neptunian objects (TNOs) and an excess in microlensing events in the 5-year Optical Gravitational Lensing Experiment data set can be simultaneously explained by a new population of astrophysical bodies with mass several times that of the Earth (M⊕). We take these objects to be primordial black holes (PBHs) and point out the orbits of TNOs would be altered if one of these PBHs was captured by the Solar System, inline with the Planet 9 hypothesis. Capture of a free floating planet is a leading explanation for the origin of Planet 9, and we show that the probability of capturing a PBH instead is comparable. The observational constraints on a PBH in the outer Solar System significantly differ from the case of a new ninth planet. This scenario could be confirmed through annihilation signals from the dark matter microhalo around the PBH.

I hope that this is helpful for you.


---

