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S/2015 (136472) 1

Makemake and its moon S/2015 (136472) 1 (arrow)
Discovery
Discovered by	HST/Wide Field Camera 3 Alex H. Parker Marc W. Buie William M. Grundy Keith S. Noll
Discovery date	April 2015
Designations
Alternative names	MK2 (unofficial)
Orbital characteristics
Semi-major axis	>21000 km
Eccentricity	unknown
Orbital period (sidereal)	>12.4 days
Inclination	63°–87° wrt ecliptic (prograde)
Satellite of	Makemake
Physical characteristics
Mean diameter	~175 km (4% albedo; best fit) 175–250 km (2–4% albedo)
Albedo	0.04 (best fit) 0.02–0.04
Apparent magnitude	25.0
Absolute magnitude (H)	7.89±0.04

S/2015 (136472) 1, unofficially nicknamed MK2 by the discovery team, is the only known moon of the trans-Neptunian dwarf planet Makemake. It is estimated to be 175 km (110 mi) in diameter and has a semi-major axis of at least 21,000 km (13,000 mi) from Makemake. Its orbital period is at least 12 days if it has a circular orbit. Observations leading to its discovery occurred in April 2015, using the Hubble Space Telescope's Wide Field Camera 3, and its discovery was announced on 26 April 2016.

Physical characteristics

S/2015 (136472) 1 is extremely faint, with an apparent magnitude of 25 in visible light. The satellite is 1,300 times fainter than Makemake, which corresponds to a magnitude difference of 7.80 between it and Makemake.

Prior to the discovery S/2015 (136472) 1, measurements of Makemake's far-infrared thermal emission by the Spitzer and Herschel space telescopes showed that the dwarf planet emitted more thermal radiation than expected for its size and brightness in visible light. This led astronomers to suspect that Makemake must have extra dark surface area that is contributing to this excess thermal emission. Makemake does not exhibit significant variations in brightness as it rotates, which leaves the possibilities that some of this dark surface area may either be uniformly distributed on Makemake's surface or is located on satellites orbiting Makemake. The discovery of S/2015 (136472) 1 and Makemake's slow rotation lends credibility to the hypothesis that Makemake's excess thermal emission is largely due to one or potentially more satellites.

Assuming S/2015 (136472) 1 has a uniformly dark surface, the satellite has a geometric albedo or visible light reflectivity of 2–4%, which makes it as dark as charcoal. The satellite is exceptionally dark compared to Makemake's geometric albedo of 82%; this may be because the satellite's gravity is too weak to hold on to bright, volatile ices as they sublimate off the satellite's surface into space. S/2015 (136472) 1 is estimated to have a diameter between 175–250 km (109–155 mi), based on its geometric albedo and brightness in visible light. Within this range, S/2015 (136472) 1's diameter is most likely 175 km (109 mi) for a geometric albedo of 4%. If S/2015 (136472) 1 has the same density as Makemake, then it would contribute less than 0.2% of the total system mass.

Additional satellites of Makemake

S/2015 (136472) 1 alone cannot be responsible for all of Makemake's excess thermal emission because its diameter and surface area only accounts for 20–30% of the total dark surface area. Even the maximum possible diameter of S/2015 (136472) 1 is too small to account for all of this dark surface area. Furthermore, S/2015 (136472) 1 is too small to have tidally slowed down Makemake to its current rotation period. These details indicate that there may be another large, undiscovered satellite orbiting Makemake. A satellite that is responsible for Makemake's slow rotation would have to be 3–5% of Makemake's mass and 400–550 km (250–340 mi) in diameter, which would be large enough to account for the remaining dark surface area. If this undiscovered satellite's orbital period is equal to Makemake's rotation period, then it would have to be closely orbiting Makemake at a distance of 3,000–5,000 km (1,900–3,100 mi), which make it undetectable to current telescopes like Hubble. It is possible that this undiscovered satellite has an irregular shape, which could account for most of Makemake's rotational brightness variability.

Orbit

Alex Parker, the leader of the team that performed the analysis of the discovery images at the Southwest Research Institute, said that from the discovery images, MK2's orbit appears to be aligned edge-on to Earth-based observatories. This would make it difficult to detect because it would be lost in Makemake's glare much of the time, which, along with its dark surface, would contribute to previous surveys failing to observe it. Observations taken in 2018 and 2019 may be enough to determine whether the orbit is close to circular, which would suggest that MK2 was formed by an ancient impact event, or if it is significantly eccentric, which would suggest that it was captured.

Name

As of 2024, the satellite has no official name. The designation S/2015 (136472) 1 is the satellite's provisional designation, with S/ indicating "satellite", 2015 being the satellite's year of discovery, and 1 being the satellite's order of discovery in that year. (136472) is Makemake's minor-planet number.

The nickname 'MK2' simply means object 2 in the Makemake system. A permanent name may be chosen from an associated figure in the mythology of Easter Island.

Notes

1. ^ For a nominal absolute magnitude of 7.8, a geometric albedo of 0.02 gives a diameter of 250 km (160 mi) while a geometric albedo of 0.04 gives a diameter of 175 km (109 mi).
2. Given S/2015 (136472) 1's diameter of ${\displaystyle D_{MK2}=175{\text{ km}}}$ (radius ${\displaystyle R_{MK2}=87.5{\text{ km}}}$) and Makemake's dark terrain area-equivalent diameter of ${\displaystyle D_{dark}=310{\text{ to }}380{\text{ km}}}$ (radius ${\displaystyle R_{dark}=155{\text{ to }}190{\text{ km}}}$)), the ratio of S/2015 (136472) 1's surface area over the dark terrain surface area is ${\displaystyle {\frac {4\pi R_{MK2}^{2}}{4\pi R_{dark}^{2}}}}$, which gives 21–31%.

References

1. ^ Parker, A. H.; Buie, M. W.; Grundy, W. M.; Noll, K. S. (25 April 2016). "Discovery of a Makemakean Moon". The Astrophysical Journal. 825 (1): L9. arXiv:1604.07461. Bibcode:2016ApJ...825L...9P. doi:10.3847/2041-8205/825/1/L9. S2CID 119270442.
2. ^ "HubbleSite – NewsCenter – Hubble Discovers Moon Orbiting the Dwarf Planet Makemake (04/26/2016) – The Full Story". hubblesite.org. Retrieved 14 July 2022.
3. ^ William M. Grundy (13 February 2020). "Makemake (136472 2005 FY9)". Lowell Observatory. Retrieved 20 February 2020.
4. ^ Parker, A. (2 May 2016). "A Moon for Makemake". Planetary Society blogs. Planetary Society. Retrieved 2 May 2016.
5. ^ Green, Daniel W. E. (26 April 2016). "S/2015 (136472) 1". Central Bureau Electronic Telegram (4275). Central Bureau for Astronomical Telegrams. Bibcode:2016CBET.4275....1P.
6. ^ Mike Wall (26 April 2016). "Distant Dwarf Planet Makemake Has Its Own Moon". Space.com. Retrieved 26 April 2016.
7. ^ T. A. Hromakina; I. N. Belskaya; Yu. N. Krugly; V. G. Shevchenko; J. L. Ortiz; P. Santos-Sanz; R. Duffard; N. Morales; A. Thirouin; R. Ya. Inasaridze; V. R. Ayvazian; V. T. Zhuzhunadze; D. Perna; V. V. Rumyantsev; I. V. Reva; A. V. Serebryanskiy; A. V. Sergeyev; I. E. Molotov; V. A. Voropaev; S. F. Velichko (9 April 2019). "Long-term photometric monitoring of the dwarf planet (136472) Makemake". Astronomy & Astrophysics. 625: A46. arXiv:1904.03679. Bibcode:2019A&A...625A..46H. doi:10.1051/0004-6361/201935274. S2CID 102350991.
8. "JPL Small-Body Database Browser: 136472 Makemake (2005 FY₉)". NASA Jet Propulsion Laboratory (2019-05-12 last obs). Archived from the original on 16 June 2019. Retrieved 20 February 2020.
9. "Naming of Astronomical Objects". International Astronomical Union. Retrieved 8 July 2017.
10. International Astronomical Union (19 July 2008). "Fourth dwarf planet named Makemake" (Press release). International Astronomical Union (News Release – IAU0806). Archived from the original on 23 July 2008. Retrieved 20 July 2008.

External links

• MPEC 2016-H46 : S/2015 (136472) 1, Minor Planet Center, 26 April 2016

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• Makemake
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