What is the ninth planet on this image from the Voyager Golden Record?

Question

Looking through the content of the Voyager Golden Record, I came across these two images:

schematic of the solar system, part 1

schematic of the solar system, part 2

The first nine circles obviously represent the sun and the eight planets of the solar system. I assume the numbers below the planets to be:

1. the planet's diameter,
2. its average distance from the sun,
3. its mass relative to Earth (1 e is defined as 6x10^27 g in a previous image),
4. the duration of one sidereal day relative to an Earth day.

A couple of spot checks confirmed to me that this data is consistent with our current knowledge.

This still leaves the identity of the tenth circle as a mystery. At first I thought it could be Pluto, but the measurements seem very off: both the diameter and the mass are roughly equivalent to Earth's, and while it's possible that astronomers were wildly off at the time, according to Wikipedia Pluto's mass had already been estimated as 1/100 that of Earth one year before the Voyager launch. What's even more confusing is that this ninth planet's average distance from the sun is only 591 million kilometers, placing it between Mars and Jupiter.

So, is this ninth planet Pluto, and if not, what is it?

Answer 1

Given historical context, I can map three of the four parameters of the mystery planet on those of Pluto, given what was known about Pluto at the time of the production of the golden record in 1977. Hence, I'm reasonably certain that the mystery planet is supposed to be Pluto, although I cannot explain the fourth parameter.

In summary: the late 1960s and the early 1970s saw much progress in estimates of Pluto's parameters, and the "Mystery planet" parameters seem to be (generous) upper bounds of what was considered certain at the time.

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Planet diameter and mass

• Mystery planet diameter: 14000 km
• Mystery planet mass: 0.9 Earth mass

Pluto's diameter has been estimated by direct observations:

• 1950: estimated by Kuiper via direct observations at 0.46 times Earth's diameter, about 5800 km.
• 1966: star occultation by Pluto in a number of independent observations is combined by Halliday et al. to derive an upper bound on its diameter of 6800 km.
• 1978: Charon discovery by observing that Pluto "images were consistently elongated"; Pluto-Charon system estimated at about 15000-20000 km, Pluto's mass estimate down to 0.001 Earth mass.
• 1992: Pluto diameter estimate down to 2300 km; Pluto-Charon system estimated at 19600 km diameter.
• 2017: estimated at 2366 km after New Horizons flyby

While it may seem obvious that by 1977 the size of Pluto was already established at much less than the 14000 km diameter of our mystery planet, this is not the case, due to the discrepancy with its estimated mass. Progress in observations of the outer planets in the 1945-1980 time frame led to many revisions in size and mass estimates of Pluto, many of which contradicted each other. Note that at that time, Pluto's mass was estimated from its influence on Neptune's orbit, but data on that was rare, since Neptune had yet to complete a full orbit at that time and little was known about its interaction with Pluto.

Kuiper's observation on 4-5 November 1949: 0.4 arc seconds diameter. According to NASA Horizons, Pluto was at 36.46 AU, which gives a diameter of 10577 km (!). This would be consistent with then-current estimates of Pluto's mass of approximately 0.8 - 1.0 Earth masses. His observation on 22 March 1950 however resulted in an estimated diameter of 0.23 ± 0.01 arc seconds. Pluto was at 35.56 AU, corresponding to a diameter of 5673 to 6189 km. This lead him to a revised mass estimate of about 0.1 Earth mass, which Kuiper notes to be at odds with observations on Neptune. He admits to have no explanation.

In 1966, the "best" estimate of Pluto's mass is still 0.9 Earth mass, as noted by Halliday et al. They provide an upper bound on on Pluto's diameter of 6800 km, but also note that this results in an unrealistically high density given the 0.9 Earth mass. A more realistic mass estimate would be around 0.14 Earth mass, but the authors also note that this does not match with observational data on Neptune's perturbations. Since Neptune will not come near Pluto for a number of centuries, they hope that observations on Uranus "close" encounter in 1967 can provide insights on what is the right mass estimate.

Duncombe et al. in 1968 and Seidelmann et al. in 1971 take a numerical approach an estimating Pluto's mass. At that time, a diameter of about 6400 km seems to have been widely accepted in the community. The papers arrive at the estimate of Pluto's mass by determining what mass fits the available observations best (least square fit) and conclude that a mass of about 0.1 Earth masses fits the data best. However, in both papers the authors note that observations are not yet sufficient to arrive at a conclusive answer (again, note that in 1971 Neptune had not yet even completed one orbit since its discovery). Note that both papers depend on continuing observations of orbits of Saturn, Uranus and Neptune and use updated mass estimates for those planets to reinterpret results on mass estimations for Pluto.

Only in 1978, after the Voyager launch, Charon was discovered and Pluto's mass estimate was revised to 0.0017 Earth masses. The discovery of Charon allow to resolve many of the conflicts in parameter estimates, but this came too late for the Voyager golden record.

The provided "Mystery planet" parameters of 14000 km diameter and 0.9 Earth masses are (somewhat generous) upper bounds of rapidly changing knowledge of Pluto's parameters in the late 1960s and early 1970s, accommodating contradicting estimates on size and mass. Hence, I conclude that these parameter values are consistent with Pluto at that time in history.

Distance to the sun

• Mystery planet: 591e6 km
• Pluto: average 39.5 au = 591e7 km

As snoopy mentions in his answer, I have no other explanation that that this is a typo, albeit a silly one.

Duration of one sidereal day

• Mystery planet: 0.7 Earth sidereal day = 16.8 hours
• Pluto: 6 days, 9 hours.

Only in 1974 it was conclusively determined that the rotational period of Pluto was 6.38 days. Up to then, some doubt existed, as the data could be mapped onto a rotational period of 1.18 days or 6.39 days. Note however that Walker et al. had established the same number of 6.39 days already in 1955.

If the mystery planet is Pluto, the only explanation I can think of is that the parameter was supposed to read 7 (as a rounded-up value for 6.39), but that seems far fetched.

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References:

• G.P. Kuiper, "The diameter of Pluto", 1950
• M.F. Walker et al. "A photometric determination of the rotational period of Pluto", 1955
• I. Halliday et al., "An upper limit for the diameter of Pluto", 1966
• R.L. Duncombe et al., "Orbit of Neptune and the mass of Pluto", 1968
• P.K. Seidelmann et al., "Determination of the mass of Pluto", 1971
• J.S. Neff et al., "An investigation of the rotational period of the planet Pluto", 1974
• J.W. Christy et al., "The satellite of Pluto", 1978
• S.A. Stern, "The Pluto-Charon system", 1992
• F. Nimmo et al., "Mean radius and shape of Pluto and Charon from New Horizons images", 2016

Answer 2

So, this is VERY interesting. I have a Voyager golden record vinyl set that has a reference book included. In it, it has various images included with the record, among which are the same schematics and units of measurement table (also (c) Drake). The numbers appear to be identical to the ones you posted. I did a 'mystery planet' comparison to Pluto to see how the numbers really stack up (numbers from https://nssdc.gsfc.nasa.gov/planetary/factsheet/ ):

Catagory	Mystery Planet	Pluto	Factor Off
Diameter	14000 km	2370 km	6
Semi-Major Axis	591E6 km	5906E6 km	10
(planet mass)/(Earth Mass)	9/10	2/1000	~5E2
Rotation Period	16.8 h	(-) 153 h (its retrograde)	~10

So--some interesting things here.

Diameter:

The diameter of Uranus and Neptune are off by approx 12% from today's values. Diameter would have probably been one of the harder parameters to determine for a planet so small and so far away as no orbital parameters depend on it. So being off by a factor of six isn't too big of a deal to me (in grad school, we always said anything within a factor of 10 is close enough).

Semi-Major Axis (aka, distance from sun):

This is off by a factor of ten, which is significant. As you pointed out this drops our mystery planet in the asteroid belt. There is no way Drake, Sagan et al thought that something so massive would be hanging out in the belt. As another commenter posted, the total mass of the belt is < 5% of a lunar mass. (Contrary to what Star Wars and other sci-fi would have you believe, the asteroid belts are largely empty space, and the vast majority of objects there are hardly a meter in diameter. The airspace above the US is MUCH more dense with planes than the belt is with asteroids.) What is also interesting about the distance row, is the unit is NOT km. it's 'x10E6 km' And given the fact that if this was a typo, the printed number would be within 1/10 of one percent of the accepted value today. I'm willing to accept the possibility that this number was a typo with the scientific notation built in to the unit; its easy to carry the exponent and loose track of digits. There have been much larger NASA's blunders than leaving off a '0' from a number in our first tangible communication attempt with an extraterrestrial race.

Planet mass to Earth mass ratio:

This is the biggest head scratcher. Given that the best way to determine the mass of a planet is to use the orbital period of a satellite, and Charon wasn't discovered until 1978, I would expect this number to have a pretty large error, but not THIS big. Ive tried various applications of Kepler's 3rd and then Newtons version. Maybe someone can also try it in parallel, as the little guy in my calculator is known to give me wrong answers sometimes. I've tried carrying the assumed distance 'error' from above with Pluto's actual orbital period. I calculated what Pluto orbital period would be at the error distance (~3.95 AU semi-major axis corresponds to an orbit of 7.8 yrs) and used THOSE numbers to get the (planet+sun) mass. Nothing I've done comes close to a mass of 90% earths. While I don't need much suspension of belief to accept being a factor of 10 off due to a typo, being off by 5x10^2 is just too much. The mass of a planet isn't a major factor in the semi-major distance and orbital period since it is usually dwarfed by the mass of the sun, so it can be hard to 'pull out' Newtons version of Kepler's third. But again--this number is wayyyyy off.

Rotation period:

Also off by a factor of ten and quite honestly, the only way to really pull an objects rotation is to try to find a pattern in its light curve. With a lack of space-based observatories and ignorance of any eclipsing satellites (again, Charon wasn't discovered until 1978), I can fully believe that measurements from the best telescopes on the best nights (don't forget, at this time, astronomers were still using film or even glass plates! CCDs didn't get adapted into astronomy until the 80s) wouldn't get much better than a factor of ten for light curves from such a dim object. Interestingly enough, there is no notation to indicate the retrograde rotation of Venus or indication of Uranus's lopsided nature in Drake's illustrations.

So there it is....if someone can come up with some logical explanation for the mass ratio value, I would say these numbers do reflect Pluto, with some understandable error and a typo. But that mass just calls everything else in to question. What a great find--I will keep looking in to it, but wanted to share what I found so far.