Planetshine

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Saturn's moon Mimas is lit by Saturnshine on the right and sunshine at the top. Mimas double terminator.jpg
Saturn's moon Mimas is lit by Saturnshine on the right and sunshine at the top.
The Moon lit by earthshine, captured by the lunar-prospecting Clementine spacecraft in 1994. Clementine's camera reveals (from right to left) the Moon lit by earthshine, the Sun's glare rising over the Moon's dark limb, and the planets Saturn, Mars, and Mercury (the three dots at lower left). Plane of Ecliptic.jpg
The Moon lit by earthshine, captured by the lunar-prospecting Clementine spacecraft in 1994. Clementine's camera reveals (from right to left) the Moon lit by earthshine, the Sun's glare rising over the Moon's dark limb, and the planets Saturn, Mars, and Mercury (the three dots at lower left).

Planetshine is the dim illumination, by sunlight reflected from a planet, of all or part of the otherwise dark side of any moon orbiting the body. Planetlight is the diffuse reflection of sunlight from a planet, whose albedo can be measured.

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The most observed and familiar example of planetshine is earthshine on the Moon, which is most visible from the night side of Earth when the lunar phase is crescent or nearly new, [1] without the atmospheric brightness of the daytime sky. Typically, this results in the dark side of the Moon being bathed in a faint light.

Planetshine has also been observed elsewhere in the Solar System. In particular, the Cassini space probe used Saturn's shine to image portions of the planet's moons, even when they do not reflect direct sunlight. The New Horizons space probe similarly used Charon's shine to discover albedo variations on Pluto's dark side. [2]

Although using a geocentric model in 510 AD, Indian mathematician and astronomer Aryabhata was the first to correctly explain how planets and moons have no light of their own, but rather shine due to the reflection of sunlight in his Aryabhatiya . [3]

Earthshine

Diagram of planetshine Earthshine diagram.svg
Diagram of planetshine

Earthshine is visible earthlight reflected from the Moon's night side. It is also known as the Moon's ashen glow or as "the new Moon with the old Moon in her arm". [4]

Leonardo da Vinci's sketch of crescent Moon with earthshine as part of his Codex Leicester, written between 1506 and 1510 Leonardo-Earthshine.png
Leonardo da Vinci's sketch of crescent Moon with earthshine as part of his Codex Leicester, written between 1506 and 1510

Earthshine is most readily visible from a few nights before until a few nights after a new moon, during the (waxing or waning) crescent phase. When the lunar phase is new as viewed from Earth, Earth would appear nearly fully sunlit from the Moon. Sunlight is reflected from Earth to the night side of the Moon. The night side appears to glow faintly, and the entire disk of the Moon is dimly illuminated.

Earthshine reflected from the Moon, as seen through a telescope. The bright region is directly illuminated by the Sun, while the rest of the Moon is illuminated by sunlight reflected from Earth. Earth illuminates.jpg
Earthshine reflected from the Moon, as seen through a telescope. The bright region is directly illuminated by the Sun, while the rest of the Moon is illuminated by sunlight reflected from Earth.

Leonardo da Vinci explained the phenomenon in the early 16th century when he realized that both Earth and the Moon reflect sunlight at the same time. Light is reflected from Earth to the Moon and back to Earth as earthshine.

Earthshine is used to help determine the current albedo of Earth. The data are used to analyze global cloud cover, a climate factor. Oceans reflect the least amount of light, roughly 10%. Land reflects 10–25% of sunlight, and clouds reflect around 50%. Thus, the part of Earth where it is daytime and from where the Moon is visible determines how bright the earthshine on the Moon appears at any given time.

Earthshine reflected from the Moon during conjunction with Venus (left) Earthshine.jpg
Earthshine reflected from the Moon during conjunction with Venus (left)

Studies of earthshine can be used to show how the Earth's cloud cover varies over time. Preliminary results show a 6.5% dip in cloud cover between 1985 and 1997 and a corresponding increase between 1997 and 2003. This has implications for climate research, especially with regards to global warming. All clouds contribute to an increased albedo, however some clouds have a net warming effect because they trap more heat than they reflect, while others have a net cooling effect because their increased albedo reflects more radiation than they trap heat. So while the Earth's albedo is measurably increasing, the uncertainty over the amount of heat trapped means the overall effect on global temperature remains unclear. [5]

Retroreflection

Features on Earth, the Moon, and some other bodies have, to some extent, retroreflective properties. Light striking them is backscattered, or diffusely reflected preferentially back in the direction from which it has come rather than in other directions. If the light comes from the Sun, it is reflected preferentially back toward the Sun and in nearby directions. For example, when its phase is full, the Moon reflects light preferentially toward the Sun and also Earth, which is in almost the same direction. As viewed from Earth, the full Moon therefore appears brighter than it would if it scattered light uniformly in all directions. Similarly, near new moon, sunlight that has been backscattered from Earth toward the Sun and also the Moon, which is in almost the same direction, and then backscattered again from the Moon toward Earth appears much brighter, as viewed from Earth, than it would without the retroreflective effects.

The retroreflection is produced by spheres of transparent material on the reflecting surface. When it encounters a transparent sphere, light is preferentially reflected and refracted in a path, within the sphere, which exits it in the direction from which it entered. On Earth, the spheres are droplets of water in clouds. On the Moon, large numbers of solid glassy spheres are found on the surface. They are thought to have been formed from drops of molten ejecta, produced by impact events, which cooled and solidified before falling back to the surface.

Ringshine

Ringshine on Saturn as it eclipses the Sun, seen from behind from the Cassini orbiter. Saturn eclipse crop.jpg
Ringshine on Saturn as it eclipses the Sun, seen from behind from the Cassini orbiter.
Very faint ringshine can be seen on Pandora's dark side. PIA21055 - Pandora Up Close.jpg
Very faint ringshine can be seen on Pandora's dark side.

Ringshine is when sunlight is reflected by a planet's ring system onto the planet or onto the moons of the planet. This has been observed in many of the photos from the Cassini orbiter. [6]

Search for terrestrial planets

Crescent Moon and earthshine over ESO's Paranal Observatory. The crescent Moon and earthshine.jpg
Crescent Moon and earthshine over ESO's Paranal Observatory.

Scientists at NASA's Navigator Program, which specializes in the detection of terrestrial planets, have backed the launch of a Terrestrial Planet Finder (TPF) mission. [7] TPF would detect light reflected by planets orbiting stars to investigate whether they could harbor life. It would use advanced telescope technologies to look for life-marks in the light reflected from the planets, including water, oxygen and methane.

The European Space Agency has a similar mission, named Darwin, under consideration. This will also study the light from planets to detect the signatures of life. [8]

Unlike many traditional astronomical challenges, the most serious challenge for these missions is not gathering enough photons from the faint planet, but rather detecting a faint planet that is extremely close to a very bright star. For a terrestrial planet, the contrast ratio of planet to its host stars is approximately ~10−6-10−7 in the thermal infrared or ~10−9-10−10 in the optical/near infrared. For this reason, Darwin and Terrestrial Planet Finder-I will work in the thermal infrared. However, searching for terrestrial planets in the optical/near infrared has the advantage that the diffraction limit corresponds to a smaller angle for a given size telescope. Therefore, NASA is also pursuing a Terrestrial Planet Finder-C mission that will search for and study terrestrial planets using the optical (and near infrared) wavelengths. While Terrestrial Planet Finder-C aims to study the light of extrasolar planets, Darwin and Terrestrial Planet Finder-I will search for thermal infrared light that is reradiated (rather than scattered) by the planet.

In preparation for these missions, astronomers have performed detailed earthshine observations, since earthshine has the spectroscopic characteristics of light reflected by the Earth. Astronomers have paid particular attention to whether earthshine measurement can detect the red edge, a spectral feature that is due to plants. The detection of a similar spectral feature in light from an extrasolar planet would be particularly interesting, since it might be due to a light-harvesting organism. While the red edge is almost certainly the easiest way to directly detect life on Earth via earthshine observations, it could be extremely difficult to interpret a similar feature due to life on another planet, since the wavelength of the spectral feature is not known in advance (unlike most atomic or molecular spectral features).

See also

Related Research Articles

<span class="mw-page-title-main">Albedo</span> Ratio of how much light is reflected back from a body

Albedo is the fraction of sunlight that is diffusely reflected by a body. It is measured on a scale from 0 to 1. Surface albedo is defined as the ratio of radiosity Je to the irradiance Ee received by a surface. The proportion reflected is not only determined by properties of the surface itself, but also by the spectral and angular distribution of solar radiation reaching the Earth's surface. These factors vary with atmospheric composition, geographic location, and time.

<span class="mw-page-title-main">Exoplanet</span> Planet outside the Solar System

An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not then recognized as such. The first confirmation of the detection occurred in 1992. A different planet, first detected in 1988, was confirmed in 2003. As of 18 June 2024, there are 6,140 confirmed exoplanets in 4,527 planetary systems, with 969 systems having more than one planet. The James Webb Space Telescope (JWST) is expected to discover more exoplanets, and to give more insight into their traits, such as their composition, environmental conditions, and potential for life.

<span class="mw-page-title-main">Terrestrial Planet Finder</span> NASA concept study of an array of space telescopes

The Terrestrial Planet Finder (TPF) was a proposed project by NASA to construct a system of space telescopes for detecting extrasolar terrestrial planets. TPF was postponed several times and finally cancelled in 2011. There were two telescope systems under consideration, the TPF-I, which had several small telescopes, and TPF-C, which used one large telescope.

<span class="mw-page-title-main">Extraterrestrial sky</span> Extraterrestrial view of outer space

In astronomy, an extraterrestrial sky is a view of outer space from the surface of an astronomical body other than Earth.

<span class="mw-page-title-main">Coronagraph</span> Telescopic attachment designed to block out the direct light from a star

A coronagraph is a telescopic attachment designed to block out the direct light from a star or other bright object so that nearby objects – which otherwise would be hidden in the object's bright glare – can be resolved. Most coronagraphs are intended to view the corona of the Sun, but a new class of conceptually similar instruments are being used to find extrasolar planets and circumstellar disks around nearby stars as well as host galaxies in quasars and other similar objects with active galactic nuclei (AGN).

<span class="mw-page-title-main">Night sky</span> Appearance of the sky in a clear night

The night sky is the nighttime appearance of celestial objects like stars, planets, and the Moon, which are visible in a clear sky between sunset and sunrise, when the Sun is below the horizon.

Darwin was a suggested ESA Cornerstone mission which would have involved a constellation of four to nine spacecraft designed to directly detect Earth-like planets orbiting nearby stars and search for evidence of life on these planets. The most recent design envisaged three free-flying space telescopes, each three to four metres in diameter, flying in formation as an astronomical interferometer. These telescopes were to redirect light from distant stars and planets to a fourth spacecraft, which would have contained the beam combiner, spectrometers, and cameras for the interferometer array, and which would have also acted as a communications hub. There was also an earlier design, called the "Robin Laurance configuration," which included six 1.5 metre telescopes, a beam combiner spacecraft, and a separate power and communications spacecraft.

<span class="mw-page-title-main">Interplanetary medium</span> Material which fills the Solar System

The interplanetary medium (IPM) or interplanetary space consists of the mass and energy which fills the Solar System, and through which all the larger Solar System bodies, such as planets, dwarf planets, asteroids, and comets, move. The IPM stops at the heliopause, outside of which the interstellar medium begins. Before 1950, interplanetary space was widely considered to either be an empty vacuum, or consisting of "aether".

<span class="mw-page-title-main">Sudarsky's gas giant classification</span>

Sudarsky's classification of gas giants for the purpose of predicting their appearance based on their temperature was outlined by David Sudarsky and colleagues in the paper Albedo and Reflection Spectra of Extrasolar Giant Planets and expanded on in Theoretical Spectra and Atmospheres of Extrasolar Giant Planets, published before any successful direct or indirect observation of an extrasolar planet atmosphere was made. It is a broad classification system with the goal of bringing some order to the likely rich variety of extrasolar gas-giant atmospheres.

<span class="mw-page-title-main">New Worlds Mission</span> Proposed occulter

The New Worlds Mission is a proposed project comprising a large occulter flying in formation with a space telescope designed to block the light of nearby stars in order to observe their orbiting exoplanets. The observations could be taken with an existing space telescope or a dedicated visible light optical telescope optimally designed for the task of finding exoplanets. A preliminary research project was funded from 2005 through 2008 by NASA Institute for Advanced Concepts (NIAC) and headed by Webster Cash of the University of Colorado at Boulder in conjunction with Ball Aerospace & Technologies Corp., Northrop Grumman, Southwest Research Institute and others. Since 2010 the project has been looking for additional financing from NASA and other sources in the amount of roughly US$3 billion including its own four-meter telescope. If financed and launched, it would operate for five years.

A nuller is an optical tool used to block a strong source so that fainter signals near that source can be observed. An example of a nuller is being employed on the Keck Interferometer. This causes the light from a star to destructively interfere, effectively cancelling the star's image. As a result, the faint light from a ring of dust orbiting the star can then be detected. This project is part of a scientific effort to detect and observe nearby planets.

<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 January 2024 have been observed directly, with even fewer being resolved from their host star.

<span class="mw-page-title-main">Extraterrestrial atmosphere</span> Area of astronomical research

The study of extraterrestrial atmospheres is an active field of research, both as an aspect of astronomy and to gain insight into Earth's atmosphere. In addition to Earth, many of the other astronomical objects in the Solar System have atmospheres. These include all the giant planets, as well as Mars, Venus and Titan. Several moons and other bodies also have atmospheres, as do comets and the Sun. There is evidence that extrasolar planets can have an atmosphere. Comparisons of these atmospheres to one another and to Earth's atmosphere broaden our basic understanding of atmospheric processes such as the greenhouse effect, aerosol and cloud physics, and atmospheric chemistry and dynamics.

<span class="mw-page-title-main">Earthlight (astronomy)</span> Light reflected from the Earth

Earthlight is the diffuse reflection of sunlight reflected from Earth's surface and clouds. Earthshine, also known as the Moon's ashen glow, is the dim illumination of the otherwise unilluminated portion of the Moon by this indirect sunlight. Earthlight on the Moon during the waxing crescent is called "the old Moon in the new Moon's arms", while that during the waning crescent is called "the new Moon in the old Moon's arms".

<span class="mw-page-title-main">Kepler-7b</span> Hot Jupiter orbiting Kepler-7

Kepler-7b is one of the first five exoplanets to be confirmed by NASA's Kepler spacecraft, and was confirmed in the first 33.5 days of Kepler's science operations. It orbits a star slightly hotter and significantly larger than the Sun that is expected to soon reach the end of the main sequence. Kepler-7b is a hot Jupiter that is about half the mass of Jupiter, but is nearly 1.5 times its size; at the time of its discovery, Kepler-7b was the second most diffuse planet known, surpassed only by WASP-17b. It orbits its host star every five days at a distance of approximately 0,06 AU. Kepler-7b was announced at a meeting of the American Astronomical Society on January 4, 2010. It is the first extrasolar planet to have a crude map of cloud coverage.

In astronomy, a phase curve describes the brightness of a reflecting body as a function of its phase angle. The brightness usually refers the object's absolute magnitude, which, in turn, is its apparent magnitude at a distance of one astronomical unit from the Earth and Sun.

<span class="mw-page-title-main">Kepler-10b</span> Terrestrial exoplanet orbiting Kepler-10

Kepler-10b is the first confirmed terrestrial planet to have been discovered outside the Solar System by the Kepler Space Telescope. Discovered after several months of data collection during the course of the NASA-directed Kepler Mission, which aims to discover Earth-like planets crossing in front of their host stars, the planet's discovery was announced on January 10, 2011. Kepler-10b has a mass of 3.72±0.42 Earth masses and a radius of 1.47 Earth radii. However, it lies extremely close to its star, Kepler-10, and as a result is too hot to support life as we know it. Its existence was confirmed using measurements from the W.M. Keck Observatory in Hawaii.

<span class="mw-page-title-main">Technosignature</span> Property that provides scientific evidence for the presence of technology

Technosignature or technomarker is any measurable property or effect that provides scientific evidence of past or present technology. Technosignatures are analogous to biosignatures, which signal the presence of life, whether intelligent or not. Some authors prefer to exclude radio transmissions from the definition, but such restrictive usage is not widespread. Jill Tarter has proposed that the search for extraterrestrial intelligence (SETI) be renamed "the search for technosignatures". Various types of technosignatures, such as radiation leakage from megascale astroengineering installations such as Dyson spheres, the light from an extraterrestrial ecumenopolis, or Shkadov thrusters with the power to alter the orbits of stars around the Galactic Center, may be detectable with hypertelescopes. Some examples of technosignatures are described in Paul Davies's 2010 book The Eerie Silence, although the terms "technosignature" and "technomarker" do not appear in the book.

<span class="mw-page-title-main">Spectro-Polarimetric High-Contrast Exoplanet Research</span>

Spectro-Polarimetric High-contrast Exoplanet REsearch (VLT-SPHERE) is an adaptive optics system and coronagraphic facility at the Very Large Telescope (VLT). It provides direct imaging as well as spectroscopic and polarimetric characterization of exoplanet systems. The instrument operates in the visible and near infrared, achieving exquisite image quality and contrast over a small field of view around bright targets.

References

  1. ↑ "Earthshine". NASA. 30 October 2020.
  2. ↑ Lauer, Todd R.; Spencer, John R.; Bertrand, Tanguy; Beyer, Ross A.; Runyon, Kirby D.; White, Oliver L.; Young, Leslie A.; Ennico, Kimberly; MacKinnon, William B.; Moore, Jeffrey M.; Olkin, Catherine B.; Stern, S. Alan; Weaver, Harold A. (20 October 2021). "The Dark Side of Pluto". The Planetary Science Journal. 2 (214): 214. arXiv: 2110.11976 . Bibcode:2021PSJ.....2..214L. doi: 10.3847/PSJ/ac2743 . S2CID   239047659.
  3. ↑ "Aryabhatta | 10 Major Contributions And Achievements | Learnodo Newtonic" . Retrieved 25 December 2021.
  4. ↑ e.g. in the Scots ballad of Sir Patrick Spens
  5. ↑ Shiga, David (25 June 2004). "Moon Study Tracks Changes in Earth's Cloud Cover". Sky & Telescope .[ permanent dead link ]
  6. ↑ "Cassini Solstice Mission: Saturn by Ringshine". NASA. Archived from the original on 1 January 2015. Retrieved 25 June 2011.
  7. ↑ "Planet Quest: Missions - Terrestrial Planet Finder". Archived from the original on 18 February 2008. Retrieved 3 March 2008.
  8. ↑ "Darwin overview".

Rush – Earthshine from album Vapor Trails (Remastered 2013). Music Lee, Lifeson. Lyrics Peart

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