Planetary mass


Planetary mass is a measure of the mass of a planet-like object. Within the Solar System, planets are usually measured in the astronomical system of units, where the unit of mass is the solar mass, the mass of the Sun. In the study of extrasolar planets, the unit of measure is typically the mass of Jupiter for large gas giant planets, and the mass of Earth for smaller rocky terrestrial planets.
The mass of a planet within the Solar System is an adjusted parameter in the preparation of ephemerides. There are three variations of how planetary mass can be calculated:
The choice of solar mass,, as the basic unit for planetary mass comes directly from the calculations used to determine planetary mass. In the most precise case, that of the Earth itself, the mass is known in terms of solar masses to twelve significant figures: the same mass, in terms of kilograms or other Earth-based units, is only known to five significant figures, which is less than a millionth as precise.
The difference comes from the way in which planetary masses are calculated. It is impossible to "weigh" a planet, and much less the Sun, against the sort of mass standards which are used in the laboratory. On the other hand, the orbits of the planets give a great range of observational data as to the relative positions of each body, and these positions can be compared to their relative masses using Newton's law of universal gravitation. To convert these relative masses to Earth-based units such as the kilogram, it is necessary to know the value of the Newtonian gravitational constant, G. This constant is remarkably difficult to measure in practice, and its value is only known to a precision of one part in ten-thousand.
The solar mass is quite a large unit on the scale of the Solar System: 1.9884 kg. The largest planet, Jupiter, is 0.09% the mass of the Sun, while the Earth is about three millionths of the mass of the Sun. Various different conventions are used in the literature to overcome this problem: for example, inverting the ratio so that one quotes the planetary mass in the 'number of planets' it would take to make up one Sun. Here, we have chosen to list all planetary masses in 'microSuns' – that is the mass of the Earth is just over three 'microSuns', or three millionths of the mass of the Sun – unless they are specifically quoted in kilograms.
When comparing the planets among themselves, it is often convenient to use the mass of the Earth as a standard, particularly for the terrestrial planets. For the mass of gas giants, and also for most extrasolar planets and brown dwarfs, the mass of Jupiter is a convenient comparison.
PlanetMercuryVenusEarthMarsJupiterSaturnUranusNeptune
Earth mass 0.05530.81510.11317.895.214.617.2
Jupiter mass 0.000 170.002 560.003 150.000 3410.2990.0460.054

Planetary mass and planet formation

The mass of a planet has consequences for its structure by having a large mass, especially while it is in the hand of process of formation. A body which is more than about one ten-thousandth of the mass of the Earth can overcome its compressive strength and achieve hydrostatic equilibrium: it will be roughly spherical, and since 2006 has been classified as a dwarf planet if it orbits around the Sun. Smaller bodies like asteroids are classified as "small Solar System bodies".
A dwarf planet, by definition, is not massive enough to have gravitationally cleared its neighbouring region of planetesimals: it is not known quite how large a planet must be before it can effectively clear its neighbourhood, but one tenth of the Earth's mass is certainly sufficient.
The smaller planets retain only silicates, and are terrestrial planets like Earth or Mars, though multiple-ME super-Earths have been discovered. The interior structure of rocky planets is mass-dependent: for example, plate tectonics may require a minimum mass to generate sufficient temperatures and pressures for it to occur.
If the protoplanet grows by accretion to more than about, its gravity become large enough to retain hydrogen in its atmosphere. In this case, it will grow into a gas giant. If the planet then begins migration, it may move well within its system's frost line, and become a hot Jupiter orbiting very close to its star, then gradually losing small amounts of mass as the star's radiation strips its atmosphere.
The theoretical minimum mass a star can have, and still undergo hydrogen fusion at the core, is estimated to be about, though fusion of deuterium can occur at masses as low as 13 Jupiters.

Values from the DE405 ephemeris

The DE405/LE405 ephemeris from the Jet Propulsion Laboratory is a widely used ephemeris dating from 1998 and covering the whole Solar System. As such, the planetary masses form a self-consistent set, which is not always the case for more recent data.

Earth mass and lunar mass

Where a planet has natural satellites, its mass is usually quoted for the whole system, as it is the mass of the whole system which acts as a perturbation on the orbits of other planets. The distinction is very slight, as natural satellites are much smaller than their parent planets.
The Earth and the Moon form a case in point, partly because the Moon is unusually large in relation to its parent planet compared with other natural satellites. There are also very precise data available for the Earth–Moon system, particularly from the Lunar Laser Ranging Experiment.
The geocentric gravitational constant – the product of the mass of the Earth times the Newtonian gravitational constant – can be measured to high precision from the orbits of the Moon and of artificial satellites. The ratio of the two masses can be determined from the slight wobble in the Earth's orbit caused by the gravitational attraction of the Moon.

More recent values

The construction of a full, high-precision Solar System ephemeris is an onerous task. It is possible to construct partial ephemerides which only concern the planets of interest by "fixing" the motion of the other planets in the model. The two methods are not strictly equivalent, especially when it comes to assigning uncertainties to the results: however, the "best" estimates – at least in terms of quoted uncertainties in the result – for the masses of minor planets and asteroids usually come from partial ephemerides.
Nevertheless, new complete ephemerides continue to be prepared, most notably the EPM2004 ephemeris from the Institute of Applied Astronomy of the Russian Academy of Sciences. EPM2004 is based on separate observations between 1913 and 2003, more than seven times as many as DE405, and gave more precise masses for Ceres and five asteroids.
EPM2004Vitagliano & Stoss
Brown & Schaller
Tholen et al.
Pitjeva & Standish
Ragozzine & Brown
136199 Eris 84.0
134340 Pluto 73.224
136108 Haumea 20.1
1 Ceres4.753 4.72
4 Vesta1.344 1.35
2 Pallas1.027 1.03
15 Eunomia 0.164
3 Juno0.151
7 Iris0.063
324 Bamberga0.055

IAU best estimates (2009)

A new set of "current best estimates" for various astronomical constants was approved the 27th General Assembly of the International Astronomical Union in August 2009.
PlanetRatio of the solar mass
to the planetary mass
Planetary mass × 10−6
Mass Ref
Mercury6023.60.3.3010
Venus408.2.4.1380
Mars3098.0.6.4273
Jupiter1.954.79191.89852
Saturn3.285.5.6846
Uranus22.43.8.6819
Neptune19.51.1.02431

IAU current best estimates (2012)

The 2009 set of "current best estimates" was updated in 2012 by resolution B2 of the IAU XXVIII General Assembly.

Improved values were given for Mercury and Uranus.
PlanetRatio of the solar mass
to the planetary mass
Mercury6023.657 33
Uranus22.