October 6

A Planet Around Another Sun

1995 — Astronomers Announce the Discovery of 51 Pegasi b

Time not applicable

On October 6, 1995, two Swiss astronomers announced a discovery that changed humanity's understanding of planetary systems. Michel Mayor and Didier Queloz had detected a planet orbiting 51 Pegasi, an ordinary Sun-like star approximately 50 light-years from Earth. The planet became known as 51 Pegasi b. It was the first planet discovered orbiting a main-sequence star similar to the Sun. Astronomers had wondered about planets around other stars for centuries. Were the planets of our solar system unusual? Or did other stars have worlds of their own? Claims of extrasolar planets had appeared before, but many collapsed under further observation. By the early 1990s, astronomers had discovered planets orbiting a pulsar—the dense remnant of an exploded star—but no planet had yet been reliably detected around an ordinary Sun-like star. Mayor and Queloz were searching for such worlds at the Observatoire de Haute-Provence in southern France. They were not trying to photograph planets directly. Instead, they measured the motion of stars. A planet does not simply orbit a motionless star. Both planet and star orbit their common center of mass. Because a star is vastly more massive, its motion is small. But sufficiently precise spectroscopy can detect the star moving slightly toward and away from Earth. As the star approaches, features in its spectrum shift slightly toward shorter, bluer wavelengths. As it recedes, they shift toward longer, redder wavelengths. This is the radial-velocity method. Mayor and Queloz used an advanced spectrograph called ELODIE to measure those tiny changes. The observations of 51 Pegasi revealed a regular pattern. The star was wobbling. And the wobble repeated approximately every 4.23 days. The most convincing explanation was a planet. But the inferred planet was astonishing. 51 Pegasi b had at least roughly half the mass of Jupiter, yet it orbited its star at only about one-twentieth the Earth-Sun distance. A year on the planet lasted just over four Earth days. Nothing like it existed in our solar system. Mercury, our innermost planet, requires 88 days to orbit the Sun. Jupiter takes nearly twelve years. Planet-formation models of the period generally expected giant planets to form far from their stars, where temperatures were low enough for abundant solid material and ices to contribute to the formation of massive planetary cores. Yet here appeared to be a Jupiter-like planet almost touching its star by comparison. The discovery initially seemed so surprising that skepticism was understandable. But independent observations soon confirmed the signal. A new category of planet entered the astronomical vocabulary: the hot Jupiter. The discovery also forced astronomers to reconsider how planetary systems form and evolve. Perhaps giant planets could form farther from their stars and then migrate inward, their orbits altered through interactions with the disks of gas and dust surrounding young stars or through later gravitational interactions with other planets. If planetary systems could change dramatically after formation, then our own solar system represented only one possible architecture among many. The importance of the discovery extended much farther than one strange planet. Once astronomers knew that planets around ordinary stars could be detected, searches accelerated. New instruments achieved greater precision. Transit surveys watched for the tiny dimming produced when planets crossed in front of their stars. Space telescopes such as Kepler discovered thousands of planetary candidates. Other observatories studied planetary atmospheres. Astronomers found super-Earths, mini-Neptunes, hot Jupiters, compact multiplanet systems, circumbinary planets orbiting two stars, and worlds occupying environments unlike anything represented in our solar system. The question gradually changed. Astronomers no longer asked simply: Do other stars have planets? They began asking: What kinds of planets exist, how do planetary systems form, and how common are worlds that might support life? In 2019, the Nobel Prize in Physics recognized Mayor and Queloz for the discovery of an exoplanet orbiting a solar-type star. 51 Pegasi b was later given the official name Dimidium. The planet itself is unlikely to resemble anything habitable. Its importance lies elsewhere. It revealed that the galaxy contains planetary systems that nature assembled in ways astronomers had scarcely imagined.

Why It Matters: The discovery of 51 Pegasi b opened the modern era of exoplanet astronomy. It provided the first confirmed planet around a Sun-like main-sequence star, challenged existing ideas about planetary formation, and helped transform the search for other worlds from speculation into one of astronomy's most productive fields.

 

Also on This Day

1959 — Luna 3 Sweeps Past the Moon

Closest-approach time not established

Two days after its October 4 launch, the Soviet spacecraft Luna 3 reached the Moon on October 6, 1959. Its journey had a purpose no previous spacecraft had accomplished. Luna 3 was preparing to show humanity a part of the Moon that no human being had ever seen. Because the Moon rotates once in approximately the same time it takes to orbit Earth, nearly the same lunar hemisphere always faces us. The opposite hemisphere—the lunar far side—remained largely hidden. Luna 3's trajectory carried it past the Moon's southern polar region at an altitude of approximately 7,900 kilometers, then around behind the Moon. The flyby itself was crucial. The spacecraft's looping trajectory would place it in the correct geometry to photograph the far side after closest approach. The historic photography occurred the following day, October 7. From tens of thousands of kilometers away, Luna 3 exposed 29 photographs of the previously unseen hemisphere. The spacecraft then developed the photographic film automatically, scanned the images, and transmitted them toward Earth by radio. The pictures were crude. But for the first time, humanity could see the Moon as a complete world rather than only as the familiar face presented to Earth. The images revealed a major geological surprise. The far side contained far fewer of the broad dark volcanic plains—the maria—that dominate much of the near side. Later lunar missions would map that hemisphere with vastly greater precision. But those explorations began with Luna 3's passage around the Moon on October 6.

Why It Matters: Luna 3's October 6 flyby positioned the spacecraft for humanity's first photography of the Moon's far side the following day. The mission turned a permanently hidden hemisphere from an astronomical unknown into an observable landscape.

1990 — Ulysses Launches to Explore the Sun from a New Direction

11:47:16 UTC

On October 6, 1990, Space Shuttle Discovery lifted off from Kennedy Space Center carrying five astronauts and an unusual spacecraft named Ulysses. Its destination was the Sun. But Ulysses would not travel directly toward it. Instead, it would first go to Jupiter. At 11:47:16 UTC, Discovery began mission STS-41. The crew consisted of commander Richard Richards, pilot Robert Cabana, and mission specialists William Shepherd, Bruce Melnick, and Thomas Akers. The principal payload in Discovery's cargo bay was a joint mission of the European Space Agency and NASA. Scientists understood that most planetary exploration took place near a broad plane surrounding the Sun called the ecliptic. Earth and the major planets orbit relatively close to this plane. Consequently, spacecraft launched from Earth naturally begin their journeys within it. That created a problem for solar science. Scientists wanted to study the environment over the Sun's north and south poles. No rocket available could simply launch Ulysses directly into the steeply inclined solar orbit required. Mission designers found a more elegant solution. They would use Jupiter's gravity. About six hours after Discovery reached orbit, the crew deployed Ulysses from the Shuttle's payload bay. A powerful combination of upper stages then accelerated the spacecraft away from Earth. At the time, Ulysses departed Earth faster than any previous human-made spacecraft. It traveled outward across the solar system rather than inward toward the Sun. In February 1992, Ulysses reached Jupiter. The spacecraft passed the giant planet, and Jupiter's gravity dramatically bent its trajectory. Instead of continuing near the ecliptic, Ulysses was thrown into an orbit carrying it far above and below the plane in which the planets travel. The solar poles had become accessible. Ulysses made its first passage over the Sun's southern polar region in 1994 and its first northern polar passage in 1995. The spacecraft did not photograph the Sun in the way a conventional telescope might. Instead, its instruments sampled the solar wind, magnetic fields, energetic particles, cosmic rays, interstellar dust, and other components of the heliosphere. For the first time, scientists could examine the Sun's environment in three dimensions rather than primarily from near the ecliptic plane. Ulysses discovered that the solar wind emerging from high solar latitudes behaved differently from the slower, more variable wind found nearer the solar equator. It investigated how the Sun's magnetic field changes during the solar cycle. It studied cosmic rays entering the heliosphere. And by accident, it made an extraordinary cometary discovery. In 1996, Ulysses unexpectedly encountered the ion tail of Comet Hyakutake, even though the spacecraft was hundreds of millions of kilometers from the comet's nucleus. The encounter revealed that cometary tails could extend much farther through space than previously appreciated. Ulysses had originally been designed for a mission lasting about five years. Instead, it operated for more than 18 years, completing nearly three circuits of the Sun. Operations finally ended in 2009.

Why It Matters: Ulysses was the first mission to investigate the space environment above and below the Sun's poles. By using Jupiter's gravity to leave the ecliptic plane, it gave scientists their first broad three-dimensional survey of the heliosphere and demonstrated once again how celestial mechanics can make possible journeys that rocket power alone cannot easily accomplish.

 

The Bigger Picture

October 6 asks us to reconsider something that seems familiar. The Moon is familiar. The Sun is familiar. Other stars are familiar. But familiarity can conceal how incomplete our knowledge really is. For almost all of human history, people could see only one general hemisphere of the Moon. Luna 3 changed that. Astronomers had observed the Sun for centuries, and spacecraft had been studying it since the beginning of the Space Age. But nearly all those observations were made from close to the plane in which Earth and the other planets orbit. Ulysses changed the perspective. Then came 51 Pegasi b. The planets of our solar system had provided humanity's only detailed example of how a planetary system was organized. There were small rocky planets close to the Sun. Giant planets lived farther away. That arrangement seemed natural. Then astronomers found a giant planet racing around its star every four days. Once again, nature was telling us: the view from where we happen to stand is not the whole story. This is one of astronomy's most persistent lessons. We see the universe from one location. Earth. We inhabit one planetary system. The solar system. We orbit one ordinary star. The Sun. From that limited sample, it is remarkably easy to assume that what surrounds us is typical. Exploration repeatedly breaks that assumption. Travel behind the Moon, and its two hemispheres look surprisingly different. Leave the ecliptic, and the Sun's environment looks different from above its poles. Examine other stars closely enough, and planetary systems appear whose architecture would once have seemed almost impossible. October 6 therefore represents one of the deepest purposes of astronomy and space exploration: change the point of view, and the universe changes with it.

At a Glance

1959 — Closest-approach time not established — Luna 3 passes approximately 7,900 kilometers above the Moon's southern polar region and swings behind the Moon, positioning itself to photograph the lunar far side the following day.

1990 — 11:47:16 UTC — Space Shuttle Discovery launches on STS-41 carrying Ulysses, the ESA-NASA spacecraft that will use Jupiter's gravity to become the first mission to explore the space environment above and below the Sun's poles.

1995 — Time not applicable — Michel Mayor and Didier Queloz announce the discovery of 51 Pegasi b, the first confirmed planet found orbiting a Sun-like main-sequence star.