Extrasolar Planets

Moon containing life orbiting a giant gas planet within the habitable zone
of an unknown star. The image involves some artistic license. Gas giants orbiting
a sun-like star at distances similar to the Earth are likely to have clouds of water
ice, with white a characteristic color. These water clouds may be obscured
by higher layers of gas, primarily methane. Methane scatters blue light
weakly, so these deeper cloud regions will have a slight bluish tinge.

The first confirmed discovery of extrasolar planets (planets around another star - also called exoplanets) occured in 1992. Radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of planets around a pulsar. In 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi). This discovery was made at the Observatoire de Haute-Provence and ushered in the modern era of exoplanetary discovery.

Discovery Methods

Technological advances allowed astronomers to detect exoplanets indirectly by determining their gravitational influence on the motion of their parent stars. The planets are usually discovered by measuring the change in Doppler shift of the star's light resulting from the star orbiting a common center of mass with a companion planet. The graphic to the left demonstrates this technique.

Several extrasolar planets were detected by observing the variation in a star's apparent luminosity as a planet passed or transited in front of it. The occasional transit of Venus across the sun is an example from our own Solar System. An hypothetical transit is illustrated on the right.

The European Extrasolar Planet Encyclopedia includes a number of observational programs managed out of Versoix in France near Geneva, Switzerland. According to the Encyclopaedia, 4232 exoplanets had been discovered by the end of 2020. At the same time, NASA's Exoplanet Archive counted 4141 exoplanets confirmed, with 5075 candidate planets. The Exoplanet Archive is operated by the California Institute of Technology under contract with NASA. The difference between the two databases highlights the uncertainties involved in exoplanet detection and confirmation.

Hot Jupiter

Most discovered planets are 'Gas Giants' or 'Hot Jupiters' (similar or larger in mass to Saturn or Jupiter) orbiting very close to their sun. Given the current early stage in planet searching technology, this preponderance of giant planet discoveries in close solar orbit should not be surprising. These would be the most noticeable planetary bodies given the weak degree of sensitivity of the available instruments.

As sensitivity increases in the future and new methodologies become common, the average range in planet distance from the parent star should increase and the average observed planet size should decrease. Astronomers have discovered that terrestrial planets might form around many, if not most, of the nearby sun-like stars in our galaxy.

Planet Searchers

Lick Observatory - Seeker of exoplanets - in a winter snow, H Graem © 2019

The Automated Planet Finder on the top of Mount Hamilton in the mountains east of San Jose, California is one of the newest and less expensive exoplanet searchers. The telescope will also be used to search for optical signals coming from laser transmissions from hypothetical extraterrestrial civilizations. Sponsored by SETI (search for extraterrestrial intelligence).

The newest searcher is TESS or Transiting Exoplanet Survey Satellite, an MIT-led NASA mission which is an all-sky survey for transiting exoplanets. TESS will monitor more than 200,000 stars for temporary drops in brightness caused by planetary transits. This first-ever spaceborne all-sky transit survey will identify planets of all sizes.The WASP or Wide Angle Search for Planets is an international consortium of several academic organisations performing an ultra-wide angle search for exoplanets using transit photometry. The array of robotic telescopes aims to survey the entire sky, simultaneously monitoring many thousands of stars.

Interstellar Travel

An obvious problem with finding exoplanets, including the holy grail of a planet habitable by human beings, is the current impossibility of interstellar travel, traveling to other stars. Some investigators are actively searching for ways to solve this problem.  Progress in revolutionary propulsion physics discusses interstellar methods currently being investigated.  Tau Zero Foundation examines the possibility of Interstellar Propulsion, giving a realistic overview of the situation and details specific efforts to find answers.