Prior to flybys, scientists used mathematical laws and observation to determine the characteristics of these distant worlds.
Titan is seen here crossing in front of Saturn. It is the planet’s largest moon. Its discovery in 1655, allowed astronomers to determine Saturn’s mass. Credit: NASA/JPL-Caltech/Space Science Institute
- Planetary size was determined pre-spacecraft by measuring the planet’s angular diameter and its distance from Earth (in AU, later converted to terrestrial units after the AU was accurately measured via Venus transits).
- Planetary mass was calculated using Newton’s modification of Kepler’s Third Law, leveraging the orbital periods and distances of the planets’ moons; the moons’ masses were considered negligible compared to their parent planets.
- The discovery of the Galilean moons (Jupiter), Titan (Saturn), Titania and Oberon (Uranus), and Triton (Neptune) provided the necessary data for mass calculations via their observed orbital parameters.
- Atmospheric composition was inferred through spectroscopy: analyzing the absorption lines in the reflected sunlight from the planets’ atmospheres revealed the presence of specific chemicals, akin to identifying barcodes for each element.
Before spacecraft missions, how did astronomers determine the mass, size, and composition of the giant planets (Jupiter, Saturn, Uranus, and Neptune)?
K. Qureshi
Calgary, Alberta
Excellent question. Astronomers are often faced with seeking information about objects they cannot visit to examine. Instead, they use mathematical laws and observation to determine the characteristics of the planets.
Determining the size of a celestial object is relatively straightforward, provided that one knows the object’s distance and the angle it subtends from our viewpoint. Even though the planets Jupiter and Saturn have been known since antiquity, their distances in terrestrial units (i.e., miles or kilometers) weren’t reliably determined until 18th-century astronomers used two transits of Venus (in 1761 and 1769) to first measure the astronomical unit, or AU, in terrestrial units. One AU is defined as the mean separation between Earth and the Sun, or 93 million miles (150 million kilometers). Prior to these measurements, Kepler’s third law (1619) enabled astronomers to determine the distances to the other planets in terms of the AU, but not in terms of terrestrial units. For instance, astronomers knew Jupiter’s average heliocentric distance was 5.2 AU — 5.2 times farther from the Sun than Earth is. And once the AU was determined in miles, so too were the distances to Jupiter and all the other known solar system worlds. This information, combined with observations of the planets’ angular diameters, which are readily discernible with a telescope, yielded direct measurements of their sizes.
Ascertaining a planet’s mass requires nothing more than an orbiting body — e.g., a moon. Fortunately, the outer planets have these in abundance. Astronomers can use Newton’s 1687 modification of Kepler’s third law to determine the masses of both a parent body and a body in orbit around it: P2 = [4Ï€2/G(M1+ M2)]*a3Â
Here, P is the moon’s orbital period, G is the gravitational constant, M1 is the planet’s mass, M2 is the moon’s mass, and a is the mean separation between the planet and its moon. By measuring a moon’s orbital period and determining the separation from its planet, one can calculate the combined mass of both bodies. Fortunately, the masses of moons tend to be negligible in relation to their planets (Earth’s moon is a notable exception), so in the case of the outer planets, the measurement of both masses essentially yields the planet’s mass.
Jupiter’s four Galilean moons were discovered in 1610. Saturn’s moon Titan was discovered in 1655. William Herschel discovered Uranus in 1781 and in 1787 also discovered its moons Titania and Oberon. William Lassell discovered Neptune’s largest moon, Triton, in 1846, very soon after the planet itself was discovered. As soon as astronomers were able to measure the orbital periods of these moons, they could determine the masses of the planets around which they revolved.Â
As for composition, astronomers can identify chemicals in the atmospheres of other planets through spectroscopy, or analysis of their light broken up by wavelength, called a spectrum. When astronomers observe the Sun’s spectrum, they see a series of dark lines located at specific wavelengths. These dark lines are produced by the absorption of photons by the chemicals present in the outer regions of the Sun. Each chemical produces its own unique absorption spectrum. Think of these absorption patterns as akin to barcodes, each of which is associated with a given chemical.
We see the planets because they all reflect a portion of the sunlight they receive out into space. That reflected light, when split into a spectrum, also includes dark lines produced due to the absorption of light by chemicals in the planet’s atmosphere. This reflected spectrum yields some direct information about the chemical composition of the outer planets’ gaseous atmospheres.
Edward Herrick-Gleason
Astronomy Educator, St. John’s, Newfoundland and Labrador
