Transit Planetary

Jupsat Pro Astronomy Software

Secrets of the Deep Sky

Get Instant Access

A planetary transit involves the passage of one celestial body in front of another, much larger-diameter celestial body. In solar system astronomy, one very important example is the transit of Venus across the face of the Sun, as seen by observers on Earth. Because of orbital mechanics, observers on Earth can witness only planetary transits of Mercury and Venus. There are about 13 transits of Mercury every century (100 years), but transits of Venus are much rarer events; in fact, only seven such events have occurred since the invention of the astronomical telescope. These transits took place in: 1631, 1639, 1761, 1769, 1874, 1882, and most recently on June 8, 2004. Any person who missed observing the 2004 transit should mark his (her) astronomical calendar because the next transit of Venus takes place on June 6, 2012.

Astronomers use contacts to characterize the principal events occurring during a transit. During one of the rare transits of Venus, for example, the event begins with contact I, which is the instant the planet's disk is externally tangent to the Sun. The entire disk of Venus is first seen at contact II, when the planet is internally tangent to the Sun. During the next several hours, Venus gradually traverses the solar disk at a relative angular rate of approximately 4 arcminutes per hour. At contact III, the planet reaches the opposite limb and is once again internally tangent to the Sun. The transit ends at contact IV, when the planet limb is externally tangent to the Sun. Contacts I and II define the phase of the transit called ingress, while astronomers refer to contacts III and IV as the egress phase or simply the egress.

From celestial mechanics, transits of Venus are only possible early December and early June when Venus's nodes pass across the Sun. If Venus reaches inferior conjunction at this time, a transit occurs. As you may have noticed from the list of the historic transits of Venus, these transits show a clear pattern of recurrence and take place at intervals of 8, 121.5, 8, and 105.5 years. The next pair of Venus transits will occur more than a century from now on December 11, 2117, and December 8, 2125.

So it is probably best to make plans to observe the upcoming June 6, 2012, transit of Venus. The entire 2012 transit (that is all four contacts) will be visible from northwestern North America, Hawaii, the western Pacific, northern Asia, Japan, Korea, eastern China, the Philippines, eastern Australia, and New Zealand. Unfortunately, no portion of the 2012 transit will be visible from Portugal or southern Spain, western Africa, and the southeastern two-thirds of South America. For the parts of the world not previously mentioned, the Sun will be setting or rising while the transit is in progress—so it will not be possible to observe the complete event (that is, all four contacts).

Cirurgia Para Aumentar Estatura

This is an artist's rendering of NASA's proposed Terrestrial Planet Finder (TPF) mission—a constellation of coorbiting telescopes with the mission to look for Earthlike planets that orbit nearby stars and to study the suitability of such planets as homes for any possible life. Scientists will use (ca. 2015) spectroscopic instruments on TPF to measure relative amounts of such gases as carbon dioxide, water vapor, ozone, and methane in the atmosphere of any detected terrestrial planets. These data, called biomarkers, will help exobiologists determine whether a planet is suitable for life—or perhaps whether life already exists there. (NASA/JPL)

detection and planet confirmation. The measured orbit of the planet and the known properties of the parent star are used to determine if each planet discovered is in the continuously habitable zone (CHZ), that is, at the distance from its parent star where liquid water could exist on the surface of the planet.

The Kepler spacecraft will hunt for planets using a specialized 3-foot-(1-meter-) diameter telescope called a photometer. This instrument can measure the small changes in brightness caused by the transits. By monitoring 100,000 stars similar to the Sun for four years following the Kepler spacecraft's launch, scientists expect to find many hundreds of terrestrial-type planets.

NASA's proposed Terrestrial Planet Finder (TPF) mission will consist of a suite of two complementary space observatories: a visible-light coro-

nagraph and a midinfrared formation-flying interferometer. An interferometer consists of a collection of several (small) telescopes that function together to produce an image that is much sharper than would be possible with a single telescope. As currently planned, the Terrestrial Planet Finder coronagraph (TPF-C) should launch in 2014, and the four spacecraft making up the Terrestrial Planet Finder interferometer (TPF-I) by 2020. The combination will detect and characterize Earth-like planets around as many as 150 stars as many as up to 45 light-years away.

The science goals of the ambitious TPF mission include a survey of nearby stars in the search for terrestrial-size planets in the continuously habitable zone (CHZ). Scientists will then perform spectroscopy on the most promising candidate extrasolar planets, looking for atmospheric signatures that are characteristic of habitability or even life itself.

This artist's rendering shows how a fiery hot star and its nearby planetary companion might look close up if viewed in visible (left) and infrared (right) light. In visible light, a star shines brilliantly, overwhelming the little (visible) light that is reflected by the nearby planet. However, when viewed in the infrared portion of the electromagnetic spectrum, a star is much less blinding, and its companion planet perks up in a more readily observable thermal glow. A hot (5,700 K) yellow star, like the Sun, is brightest in the visible (yellow-light) wavelengths, while thermal radiation from a warm (300 K) planet peaks in the infrared portion of the EM spectrum. (NASA/JPL-Caltech/R. Hurt [SSC])

This artist's rendering shows how a fiery hot star and its nearby planetary companion might look close up if viewed in visible (left) and infrared (right) light. In visible light, a star shines brilliantly, overwhelming the little (visible) light that is reflected by the nearby planet. However, when viewed in the infrared portion of the electromagnetic spectrum, a star is much less blinding, and its companion planet perks up in a more readily observable thermal glow. A hot (5,700 K) yellow star, like the Sun, is brightest in the visible (yellow-light) wavelengths, while thermal radiation from a warm (300 K) planet peaks in the infrared portion of the EM spectrum. (NASA/JPL-Caltech/R. Hurt [SSC])

Was this article helpful?

0 0
Angel Ascendancy

Angel Ascendancy

Be Prepared To See Massive Changes In Your Destiny Guided By The Archangels. This Book Is One Of The Most Valuable Guide To Communicate With Archangels For Life.

Get My Free Ebook


Post a comment