Astronomical transit

Last updated
Phobos transits the Sun, as viewed by the Perseverance rover on 2 April 2022

In astronomy, a transit (or astronomical transit) is the passage of a celestial body directly between a larger body and the observer. As viewed from a particular vantage point, the transiting body appears to move across the face of the larger body, covering a small portion of it. [1]

Contents

The word "transit" refers to cases where the nearer object appears smaller than the more distant object. Cases where the nearer object appears larger and completely hides the more distant object are known as occultations.

However, the probability of seeing a transiting planet is low because it is dependent on the alignment of the three objects in a nearly perfectly straight line. [2] Many parameters of a planet and its parent star can be determined based on the transit.

In the Solar System

A simulation of Io transiting Jupiter as seen from the Earth in February 2009. Io's shadow is seen on the surface of Jupiter, leading Io slightly due to the Sun and Earth not being in the same line. Jupiter-io-transit feb 10 2009.gif
A simulation of Io transiting Jupiter as seen from the Earth in February 2009. Io's shadow is seen on the surface of Jupiter, leading Io slightly due to the Sun and Earth not being in the same line.

One type of transit involves the motion of a planet between a terrestrial observer and the Sun. This can happen only with inferior planets, namely Mercury and Venus (see transit of Mercury and transit of Venus). However, because a transit is dependent on the point of observation, the Earth itself transits the Sun if observed from Mars. In the solar transit by the Moon captured during calibration of the STEREO B spacecraft's ultraviolet imaging, the Moon appears much smaller than it does when seen from Earth, because the spacecraft–Moon separation was several times greater than the Earth–Moon distance.

The term can also be used to describe the motion of a satellite across its parent planet, for instance one of the Galilean satellites (Io, Europa, Ganymede, Callisto) across Jupiter, as seen from Earth.

Although rare, cases where four bodies are lined up do happen. One of these events occurred on 27 June 1586, when Mercury transited the Sun as seen from Venus at the same time as a transit of Mercury from Saturn and a transit of Venus from Saturn. [ citation needed ]

Notable observations

No missions were planned to coincide with the transit of Earth visible from Mars on 11 May 1984 and the Viking missions had been terminated a year previously. Consequently, the next opportunity to observe such an alignment will be in 2084.

On 21 December 2012, the Cassini–Huygens probe, in orbit around Saturn, observed the planet Venus transiting the Sun. [3]

On 3 June 2014, the Mars rover Curiosity observed the planet Mercury transiting the Sun, marking the first time a planetary transit has been observed from a celestial body besides Earth. [4]

Mutual planetary transits

In rare cases, one planet can pass in front of another. If the nearer planet appears smaller than the more distant one, the event is called a mutual planetary transit.

Outside the Solar System

The light curve shows the change in Luminosity of star as a result of transiting. The data was collected from the Kepler mission. Light curve of binary star Kepler-16.jpg
The light curve shows the change in Luminosity of star as a result of transiting. The data was collected from the Kepler mission.

The transit method can be used to discover exoplanets. As a planet eclipses/transits its host star it will block a portion of the light from the star. If the planet transits in-between the star and the observer the change in light can be measured to construct a light curve. Light curves are measured with a charge-coupled device. The light curve of a star can disclose several physical characteristics of the planet and star, such as density. Multiple transit events must be measured to determine the characteristics which tend to occur at regular intervals. Multiple planets orbiting the same host star can cause transit-timing variations (TTV). TTV is caused by the gravitational forces of all orbiting bodies acting upon each other. The probability of seeing a transit from Earth is low, however. The probability is given by the following equation.

[5]

where Rstar and Rplanet are the radius of the star and planet, respectively, and a is the semi-major axis. Because of the low probability of a transit in any specific system, large selections of the sky must be regularly observed in order to see a transit. Hot Jupiters are more likely to be seen because of their larger radius and short semi-major axis. In order to find Earth-sized planets, red dwarf stars are observed because of their small radius. Even though transiting has a low probability it has proven itself to be a good technique for discovering exoplanets.

In recent years, the discovery of extrasolar planets has prompted interest in the possibility of detecting their transits across their own stellar primaries. HD 209458b was the first such transiting planet to be detected.

The transit of celestial objects is one of the few key phenomena used today for the study of exoplanetary systems. Today, transit photometry is the leading form of exoplanet discovery. [5] As an exoplanet moves in front of its host star there is a dimming in the luminosity of the host star that can be measured. [6] Larger planets make the dip in luminosity more noticeable and easier to detect. Followup observations using other methods are often carried out to ensure it is a planet.

There are currently (December 2018) 2345 planets confirmed with Kepler light curves for stellar host. [7]

Exoplanets found by different search methods each year through 2018, transit method in purple. Exoplanets discovery methods chart.png
Exoplanets found by different search methods each year through 2018, transit method in purple.

Contacts

During a transit there are four "contacts", when the circumference of the small circle (small body disk) touches the circumference of the large circle (large body disk) at a single point. Historically, measuring the precise time of each point of contact was one of the most accurate ways to determine the positions of astronomical bodies. The contacts happen in the following order:

A fifth named point is that of greatest transit, when the apparent centers of the two bodies are nearest to each other, halfway through the transit. [8]

Missions

Since transit photometry allows for scanning large celestial areas with a simple procedure, it has been the most popular and successful form of finding exoplanets in the past decade and includes many projects, some of which have already been retired, others in use today, and some in progress of being planned and created. The most successful projects include HATNet, KELT, Kepler, and WASP, and some new and developmental stage missions such as TESS, HATPI, and others which can be found among the List of Exoplanet Search Projects.

HATNet

HATNet Project is a set of northern telescopes in Fred Lawrence Whipple Observatory, Arizona and Mauna Kea Observatories, HI, and southern telescopes around the globe, in Africa, Australia, and South America, under the HATSouth branch of the project. [9] These are small aperture telescopes, just like KELT, and look at a wide field which allows them to scan a large area of the sky for possible transiting planets. In addition, their multitude and spread around the world allows for 24/7 observation of the sky so that more short-period transits can be caught. [10]

A third sub-project, HATPI, is currently under construction and will survey most of the night sky seen from its location in Chile. [11]

KELT

KELT is a terrestrial telescope mission designed to search for transiting systems of planets of magnitude 8<M<10. It began operation in October 2004 in Winer Observatory and has a southern companion telescope added in 2009. [12] KELT North observes "26-degree wide strip of sky that is overhead from North America during the year", while KELT South observes single target areas of the size 26 by 26 degrees. Both telescopes can detect and identify transit events as small as a 1% flux dip, which allows for detection of planetary systems similar to those in our planetary system. [13] [14]

Kepler / K2

The Kepler satellite served the Kepler mission between 7 March 2009 and 11 May 2013, where it observed one part of the sky in search of transiting planets within a 115 square degrees of the sky around the Cygnus, Lyra, and Draco constellations. [15] After that, the satellite continued operating until 15 November 2018, this time changing its field along the ecliptic to a new area roughly every 75 days due to reaction wheel failure. [16]

TESS

TESS was launched on 18 April 2018, and is planned to survey most of the sky by observing it strips defined along the right ascension lines for 27 days each. Each area surveyed is 27 by 90 degrees. Because of the positioning of sections, the area near TESS's rotational axis will be surveyed for up to 1 year, allowing for the identification of planetary systems with longer orbital periods.

See also

Related Research Articles

<span class="mw-page-title-main">Conjunction (astronomy)</span> When two astronomical objects have the same right ascension or the same ecliptic longitude

In astronomy, a conjunction occurs when two astronomical objects or spacecraft have either the same right ascension or the same ecliptic longitude, usually as observed from Earth.

<span class="mw-page-title-main">Kepler space telescope</span> NASA satellite for exoplanetology (2009–2018)

The Kepler space telescope is a disused space telescope launched by NASA in 2009 to discover Earth-sized planets orbiting other stars. Named after astronomer Johannes Kepler, the spacecraft was launched into an Earth-trailing heliocentric orbit. The principal investigator was William J. Borucki. After nine and a half years of operation, the telescope's reaction control system fuel was depleted, and NASA announced its retirement on October 30, 2018.

<span class="mw-page-title-main">Lists of exoplanets</span>

These are lists of exoplanets. As of 28 July 2023, there are 5,483 confirmed exoplanets in 4,082 planetary systems, with 924 systems having more than one planet. Most of these were discovered by the Kepler space telescope. There are an additional 1,984 potential exoplanets from Kepler's first mission yet to be confirmed, as well as 977 from its "Second Light" mission and 4,446 from the Transiting Exoplanet Survey Satellite (TESS) mission.

<span class="mw-page-title-main">Methods of detecting exoplanets</span>

Any planet is an extremely faint light source compared to its parent star. For example, a star like the Sun is about a billion times as bright as the reflected light from any of the planets orbiting it. In addition to the intrinsic difficulty of detecting such a faint light source, the light from the parent star causes a glare that washes it out. For those reasons, very few of the exoplanets reported as of April 2014 have been observed directly, with even fewer being resolved from their host star.

<span class="mw-page-title-main">Discovery and exploration of the Solar System</span>

Discovery and exploration of the Solar System is observation, visitation, and increase in knowledge and understanding of Earth's "cosmic neighborhood". This includes the Sun, Earth and the Moon, the major planets Mercury, Venus, Mars, Jupiter, Saturn, Uranus, and Neptune, their satellites, as well as smaller bodies including comets, asteroids, and dust.

<span class="mw-page-title-main">Kepler-4b</span> Extrasolar planet in the constellation Draco

Kepler-4b, initially known as KOI 7.01, is an extrasolar planet first detected as a transit by the Kepler spacecraft. Its radius and mass are similar to that of Neptune; however, due to its proximity to its host star, it is substantially hotter than any planet in the Solar System. The planet's discovery was announced on January 4, 2010, in Washington, D.C., along with four other planets that were initially detected by the Kepler spacecraft and subsequently confirmed by telescopes at the W.M. Keck Observatory.

<span class="mw-page-title-main">Kepler-10</span> Sunlike star in the constellation Draco

Kepler-10, formerly known as KOI-72, is a Sun-like star in the constellation of Draco that lies 607 light-years from Earth. Kepler-10 was targeted by NASA's Kepler spacecraft, as it was seen as the first star identified by the Kepler mission that could be a possible host to a small, transiting exoplanet. The star is slightly less massive, slightly larger, and slightly cooler than the Sun; at an estimated 11.9 billion years in age, Kepler-10 is almost 2.6 times the age of the Sun. Kepler-10 is host to a planetary system made up of at least three planets. Kepler-10b, the first undeniably rocky planet, was discovered in its orbit after eight months of observation and announced on January 10, 2011. The planet orbits its star closely, completing an orbit every 0.8 days, and has a density similar to that of iron. The second planet, Kepler-10c, was confirmed on May 23, 2011, based on follow-up observations by the Spitzer Space Telescope. The data shows it has an orbital period of 42.3 days and has a radius more than double that of Earth, but it was initially thought to have a higher density, making it the largest and most massive rocky planet discovered as of June 2014. However, refined mass measurements have shown it to be a more typical volatile-rich planet. A third planet, Kepler-10d, was discovered in 2023 by radial velocity observations.

<span class="mw-page-title-main">Kepler-11b</span> Exoplanet orbiting Kepler-11

Kepler-11b is an exoplanet discovered around the star Kepler-11 by the Kepler spacecraft, a NASA-led mission to discover Earth-like planets. Kepler-11b is less than about three times as massive and twice as large as Earth, but it has a lower density, and is thus most likely not of Earth-like composition. Kepler-11b is the hottest of the six planets in the Kepler-11 system, and orbits more closely to Kepler-11 than the other planets in the system. Kepler-11b, along with its five counterparts, form the first discovered planetary system with more than three transiting planets—the most densely packed known planetary system. The system is also the flattest known planetary system. The discovery of this planet and its five sister planets was announced on February 2, 2011, after follow-up investigations.

<span class="mw-page-title-main">Kepler-11g</span> Extrasolar planet

Kepler-11g is an exoplanet discovered in the orbit of the sunlike star Kepler-11 by the Kepler spacecraft, a NASA satellite tasked with searching for terrestrial planets. Kepler-11g is the outermost of the star's six planets. The planet orbits at a distance of nearly half the mean distance between Earth and the Sun. It completes an orbit every 118 days, placing it much further from its star than the system's inner five planets. Its estimated radius is a little over three times that of Earth, i.e. comparable to Neptune's size. Kepler-11g's distance from the inner planets made its confirmation more difficult than that of the inner planets, as scientists had to work to exhaustively disprove all reasonable alternatives before Kepler-11g could be confirmed. The planet's discovery, along with that of the other Kepler-11 planets, was announced on February 2, 2011. According to NASA, the Kepler-11 planets form the flattest and most compact system yet discovered.

The NASA Star and Exoplanet Database (NStED) is an on-line astronomical stellar and exoplanet catalog and data service that collates and cross-correlates astronomical data and information on exoplanets and their host stars. NStED is dedicated to collecting and serving important public data sets involved in the search for and characterization of exoplanets and their host stars. The data include stellar parameters, exoplanet parameters and discovery/characterization data.

<span class="mw-page-title-main">NASA Exoplanet Archive</span> Online astronomical exoplanet catalog and data service

The NASA Exoplanet Archive is an online astronomical exoplanet catalog and data service that collects and serves public data that support the search for and characterization of extra-solar planets (exoplanets) and their host stars. It is part of the Infrared Processing and Analysis Center and is on the campus of the California Institute of Technology (Caltech) in Pasadena, CA. The archive is funded by NASA and was launched in early December 2011 by the NASA Exoplanet Science Institute as part of NASA's Exoplanet Exploration Program. In June 2019, the archive's collection of confirmed exoplanets surpassed 4,000.

<span class="mw-page-title-main">Kepler-37</span> G-type main-sequence star in the constellation Lyra

Kepler-37, also known as UGA-1785, is a G-type main-sequence star located in the constellation Lyra 209 light-years from Earth. It is host to exoplanets Kepler-37b, Kepler-37c, Kepler-37d and possibly Kepler-37e, all of which orbit very close to it. Kepler-37 has a mass about 80.3 percent of the Sun's and a radius about 77 percent as large. It has a temperature similar to that of the Sun, but a bit cooler at 5,417 K. It has about half the metallicity of the Sun. With an age of roughly 6 billion years, it is slightly older than the Sun, but is still a main-sequence star. Until January 2015, Kepler-37 was the smallest star to be measured via asteroseismology.

<span class="mw-page-title-main">Kepler-62f</span> Super-Earth orbiting Kepler-62

Kepler-62f is a super-Earth exoplanet orbiting within the habitable zone of the star Kepler-62, the outermost of five such planets discovered around the star by NASA's Kepler spacecraft. It is located about 980 light-years from Earth in the constellation of Lyra.

<span class="mw-page-title-main">Kepler-438b</span> Super-Earth orbiting Kepler-438

Kepler-438b is a confirmed near-Earth-sized exoplanet. It is likely rocky. It orbits on the inner edge of the habitable zone of a red dwarf, Kepler-438, about 472.9 light-years from Earth in the constellation Lyra. It receives 1.4 times our solar flux. The planet was discovered by NASA's Kepler spacecraft using the transit method, in which the dimming effect that a planet causes as it crosses in front of its star is measured. NASA announced the confirmation of the exoplanet on 6 January 2015.

<span class="mw-page-title-main">Kepler-442b</span> Super-Earth orbiting Kepler-442

Kepler-442b is a confirmed near-Earth-sized exoplanet, likely rocky, orbiting within the habitable zone of the K-type main-sequence star Kepler-442, about 1,206 light-years (370 pc) from Earth in the constellation of Lyra.

<span class="mw-page-title-main">Next-Generation Transit Survey</span> Ground-based robotic search for exoplanets

<span class="mw-page-title-main">Kepler-452b</span> Super-Earth exoplanet orbiting Kepler-452

Kepler-452b is a super-Earth exoplanet orbiting within the inner edge of the habitable zone of the sun-like star Kepler-452 and is the only planet in the system discovered by Kepler. It is located about 1,800 light-years (550 pc) from Earth in the constellation of Cygnus.

<span class="mw-page-title-main">Kepler-1229b</span> Super-Earth orbiting Kepler-1229

Kepler-1229b is a confirmed super-Earth exoplanet, likely rocky, orbiting within the habitable zone of the red dwarf Kepler-1229, located about 870 light years from Earth in the constellation of Cygnus. It was discovered in 2016 by the Kepler space telescope. The exoplanet was found by using the transit method, in which the dimming effect that a planet causes as it crosses in front of its star is measured.

The word "transit" refers to cases where the nearer object appears smaller than the more distant object. Cases where the nearer object appears larger and completely hides the more distant object are known as occultations.

Kepler-1708b is a Jupiter-sized exoplanet orbiting the Sun-like star Kepler-1708, located in the constellation of Cygnus approximately 5,600 light years away from Earth. It was first detected in 2011 by NASA's Kepler mission using the transit method, but was not identified as a candidate planet until 2019. In 2021, a candidate Neptune-sized exomoon in orbit around Kepler-1708b was found by astronomer David Kipping and colleagues in an analysis using Kepler transit data.

References

  1. "Definition of TRANSIT". www.merriam-webster.com. Retrieved 16 December 2018.
  2. "Transit Method | Las Cumbres Observatory". lco.global. Retrieved 27 November 2018.
  3. Cassini Spacecraft Tracks Venus Transit From Saturn , Space Coast Daily. Retrieved on 8 February 2016.
  4. Webster, Guy (10 June 2014). "Mercury Passes in Front of the Sun, as Seen From Mars". NASA .
  5. 1 2 Asher, Johnson, John (29 December 2015). How do you find an exoplanet?. Princeton, New Jersey. ISBN   9780691156811. OCLC   908083548.
  6. "Down in Front!: The Transit Photometry Method". The Planetary Society. February 2020.
  7. "Exoplanet Archive Planet Counts". exoplanetarchive.ipac.caltech.edu. Retrieved 17 December 2018.
  8. 1 2 "Transit of Venus – Safety". University of Central Lancashire. Archived from the original on 25 September 2006. Retrieved 21 September 2006.
  9. "The HATNet Exoplanet Survey". hatnet.org. Princeton University. 2018.
  10. "The HAT Exoplanet Surveys". hatsurveys.org. Archived from the original on 25 September 2021. Retrieved 16 December 2018.
  11. "The HATPI Project". hatpi.org. Retrieved 16 December 2018.
  12. Pepper, J.; Pogge, R.; Depoy, D. L.; Marshall, J. L.; Stanek, K.; Stutz, A.; Trueblood, M.; Trueblood, P. (1 July 2007). "Early Results from the KELT Transit Survey". Transiting Extrapolar Planets Workshop. 366: 27. arXiv: astro-ph/0611947 . Bibcode:2007ASPC..366...27P.
  13. "KELT-North: Method". www.astronomy.ohio-state.edu. Archived from the original on 24 January 2019. Retrieved 16 December 2018.
  14. Stassun, Keivan; James, David; Siverd, Robert; Kuhn, Rudolf B.; Pepper, Joshua (7 March 2012). "The KELT-South Telescope". Publications of the Astronomical Society of the Pacific. 124 (913): 230. arXiv: 1202.1826 . Bibcode:2012PASP..124..230P. doi:10.1086/665044. ISSN   1538-3873. S2CID   119207060.
  15. Johnson, Michele (13 April 2015). "Mission overview". NASA. Retrieved 16 December 2018.
  16. Fortney, Jonathan J.; Twicken, J. D.; Smith, Marcie; Najita, Joan R.; Miglio, Andrea; Marcy, Geoffrey W.; Huber, Daniel; Cochran, William D.; Chaplin, William J. (1 April 2014). "The K2 Mission: Characterization and Early Results". Publications of the Astronomical Society of the Pacific. 126 (938): 398. arXiv: 1402.5163 . Bibcode:2014PASP..126..398H. doi:10.1086/676406. ISSN   1538-3873. S2CID   119206652.