This artist's concept illustrates how the brightness of outbursting star FU Orionis has been slowly fading since its initial flare-up in 1936. The star is pictured with the disk of material that surrounds it. Researchers found that it has dimmed by about 13 percent at short infrared wavelengths from 2004 to 2016. This illustration represents the 2004 data that was collected with NASA's Spitzer Space Telescope. FU Orionis is a few hundred thousand years old. It is possible that when our sun was younger, it also went through a period of intense brightening followed by dimming.

This artist's concept illustrates how the brightness of outbursting star FU Orionis has been slowly fading since its initial flare-up in 1936. The star is pictured with the disk of material that surrounds it. Researchers found that it has dimmed by about 13 percent at short infrared wavelengths from 2004 (left) to 2016 (right). This illustration represents the 2016 data was collected with the Stratospheric Observatory for Infrared Astronomy (SOFIA). FU Orionis is a few hundred thousand years old. It is possible that when our sun was younger, it also went through a period of intense brightening followed by dimming.

This artist's concept illustrates how the brightness of outbursting star FU Orionis has been slowly fading since its initial flare-up in 1936. The star is pictured with the disk of material that surrounds it. Researchers found that it has dimmed by about 13 percent at short infrared wavelengths from 2004 (left) to 2016 (right). The 2004 data were collected with NASA's Spitzer Space Telescope, and the 2016 data were collected with the Stratospheric Observatory for Infrared Astronomy (SOFIA). FU Orionis is a few hundred thousand years old. It is possible that when our sun was younger, it also went through a period of intense brightening followed by dimming. These results were presented at the American Astronomical Association meeting in June 2016 in San Diego. NASA's Jet Propulsion Laboratory, Pasadena, California, manages the Spitzer Space Telescope mission for NASA. Science operations are conducted at the Spitzer Science Center at Caltech. Spacecraft operations are based at Lockheed Martin Space Systems Company, Littleton, Colorado. Data are archived at the Infrared Science Archive housed at IPAC at Caltech. Caltech manages JPL for NASA. SOFIA is a joint project of NASA and the German Aerospace Center (DLR). The aircraft is based at NASA Armstrong Flight Research Center's facility in Palmdale, California. NASA's Ames Research Center in Moffett Field, California, manages the SOFIA science and mission operations in cooperation with the Universities Space Research Association (USRA) headquartered in Columbia, Maryland, and the German SOFIA Institute (DSI) at the University of Stuttgart.

Astronomers can use light echoes to measure the distance from a star to its surrounding protoplanetary disk. This diagram illustrates how the time delay of the light echo is proportional to the distance between the star and the inner edge of the disk.

This illustration shows a star surrounded by a protoplanetary disk. Material from the thick disk flows along the stars magnetic field lines and is deposited onto the stars surface. When material hits the star, it lights up brightly. The star's irregular illumination allows astronomers to measure the gap between the disk and the star by using a technique called "photo-reverberation" or "light echoes." First, astronomers look at how much time it takes for light from the star to arrive at Earth. Then, they compare that with the time it takes for light from the star to bounce off the inner edge of the disk and then arrive at Earth. That time difference is used to measure distance, as the speed of light is constant.

The spider part of "The Spider and the Fly" nebulae, IC 417 abounds in star formation, as seen in this infrared image from NASA's Spitzer Space Telescope and the Two Micron All Sky Survey (2MASS). Located in the constellation Auriga, IC 417 lies about 10,000 light-years away. It is in the outer part of the Milky Way, almost exactly in the opposite direction from the galactic center. This region was chosen as the subject of a research project by a group of students, teachers and scientists as part of the NASA/IPAC Teacher Archive Research Program (NITARP) in 2015. A cluster of young stars called "Stock 8" can be seen at center right. The light from this cluster carves out a bowl in the nearby dust clouds, seen here as green fluff. Along the sinuous tail in the center and to the left, groupings of red point sources are also young stars. In this image, infrared wavelengths, which are invisible to the unaided eye, have been assigned visible colors. Light with a wavelength of 1.2 microns, detected by 2MASS, is shown in blue. The Spitzer wavelengths of 3.6 and 4.5 microns are green and red, respectively. Spitzer data used to create this image were obtained during the space telescope's "warm mission" phase, following its depletion of coolant in mid-2009. Due to its design, Spitzer remains cold enough to operate efficiently at two channels of infrared light. It is now in its 12th year of operation since launch. The 2MASS mission was a joint effort between the California Institute of Technology, Pasadena; the University of Massachusetts, Amherst; and NASA's Jet Propulsion Laboratory, Pasadena, California. JPL manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology in Pasadena. Spacecraft operations are based at Lockheed Martin Space Systems Company, Littleton, Colorado. Data from 2MASS and Spitzer are archived at the Infrared Science Archive housed at the Infrared Processing and Analysis Center (IPAC) at Caltech. Caltech manages JPL for NASA.

The varying brightness of an exoplanet called 55 Cancri e is shown in this plot of infrared data captured by NASA's Spitzer Space Telescope. Spitzer stared at the 55 Cancri star system for 80 hours, capturing changes in the total light of the system from both the star and the planet 55 Cancri e, as the planet orbited around the star. When the planet passes in front of the star, it blocks some of the starlight and the total light does down, as seen in the graph. When the planet passes behind the star, its light is blocked, and total light goes down again but not by as much. By analyzing light curves like these, astronomers can figure out how much light comes from just the planet alone represented by the orange, top part of the graph. The star's light remains fairly constant. They can also measure changes in the planet's light to learn about its temperature. In this case, 55 Cancri e was found to have dramatically different temperatures on each of its sides (the planet is tidally locked, so one side, the day side, always faces the star). The day side of the planet is nearly 4,400 degrees Fahrenheit (2,700 Kelvin), and the cooler, night side is 2,060 degrees Fahrenheit (1400 Kelvin).

This illustration shows one possible scenario for the hot, rocky exoplanet called 55 Cancri e, which is nearly two times as wide as Earth. New data from NASA's Spitzer Space Telescope show that the planet has extreme temperature swings from one side to the other and a possible reason for this might be the presence of lava pools. This planet is tidally locked to its star, just as our moon is to Earth, which means that one side always sizzles under the heat of its star while the other side remains in the dark. If the planet were covered in lava, then the hot, sun-facing side of the planet would have liquid lava flows, while the colder, dark side would see solidified lava rock. The hardened lava would be unable to transport heat across the planet, explaining why Spitzer detected that the cold side of the planet is much colder than the hot side. Such a lava planet, if it exists, would have dust streaming off of it, as illustrated here. Radiation and winds from the nearby star would blow off the material. Scientists say that future observations with NASA's upcoming James Webb Space Telescope should provide more details about the nature of this exotic world.

Astronomers watched an exoplanet called HD 80606b heat up and cool off during its sizzling-hot orbit around its star. The results are shown in this data plot from NASA's Spitzer Space Telescope. Spitzer measured the slight changes in infrared light coming from the distant planet and star. HD 80606b is about 190 light-years away. Its 111-day orbit takes it almost as far away from its star as Earth is from the sun, but at its closest approach, it sweeps blisteringly close to the star for a brief period. Spitzer observed the combined light from the star plus planet for a total of 80 hours, using an infrared wavelength of 4.5 microns. This was long enough to catch the hottest part of the planet's orbit, where it brightened up enough relative to the total light in the system to be more easily measured. In this chart, an illustration of the planet's orbit is shown above the data, with each disk representing the hemisphere that faces our direction on Earth. The short dip in the data reflects the period when the planet passed behind the star. For that period, only the light from the star alone was observed. This helps astronomers figure out just how bright the planet would be if we could see it by itself. The planet heats up by 1,500 degrees Fahrenheit briefly only to cool down by the same amount in less than a day. Together with earlier Spitzer observations using 8-micron infrared light, these findings help scientists understand how exotic planets like this form and evolve throughout the galaxy. NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology in Pasadena. Spacecraft operations are based at Lockheed Martin Space Systems Company, Littleton, Colorado. Data are archived at the Infrared Science Archive housed at the Infrared Processing and Analysis Center at Caltech. Caltech manages JPL for NASA.

The turbulent atmosphere of a hot, gaseous planet known as HD 80606b is shown in this simulation based on data from NASA's Spitzer Space Telescope. The planet spends most of its time far away from its star, but every 111 days, it swings extremely close to the star, experiencing a massive burst of heat. Spitzer measured the whole heating cycle of this planet, determining its coolest (less than 400 degrees Fahrenheit) and hottest (2,000 degrees Fahrenheit) temperatures. NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology in Pasadena. Spacecraft operations are based at Lockheed Martin Space Systems Company, Littleton, Colorado. Data are archived at the Infrared Science Archive housed at the Infrared Processing and Analysis Center at Caltech. Caltech manages JPL for NASA.

Astronomers have made the most detailed study yet of an extremely massive young galaxy cluster using three of NASA's Great Observatories. This multi-wavelength image shows this galaxy cluster, called IDCS J1426.5+3508 (IDCS 1426 for short), in X-rays recorded by the Chandra X-ray Observatory in blue, visible light observed by the Hubble Space Telescope in green, and infrared light detected by the Spitzer Space Telescope in red. This rare galaxy cluster, which is located 10 billion light-years from Earth, is almost as massive as 500 trillion suns. This object has important implications for understanding how such megastructures formed and evolved early in the universe. The light astronomers observed from IDCS 1426 began its journey to Earth when the universe was less than a third of its current age. It is the most massive galaxy cluster detected at such an early time. First discovered by the Spitzer Space Telescope in 2012, IDCS 1426 was then observed using the Hubble Space Telescope and the Keck Observatory to determine its distance. Observations from the Combined Array for Millimeter-wave Astronomy indicated it was extremely massive. New data from the Chandra X-ray Observatory confirm the galaxy cluster's mass and show that about 90 percent of this mass is in the form of dark matter -- the mysterious substance that has so far been detected only through its gravitational pull on normal matter composed of atoms. There is a region of bright X-ray emission (seen as blue-white) near the middle of the cluster, but not exactly at the center. The location of this "core" of gas suggests that the cluster may have had a collision or interaction with another massive system of galaxies relatively recently, perhaps within about the last 500 million years. This would cause the core to slosh around like wine in a moving glass and become offset, as it appears to be in the Chandra data. Such a merger would not be surprising, given that astronomers are observing IDCS 1426 when the universe was only 3.8 billion years old. Scientists think that, in order for such an enormous structure to form so rapidly, mergers with smaller clusters would likely play a role in the large cluster's growth. In addition, while still extremely hot, the bright core contains cooler gas than its surroundings. This is the most distant galaxy cluster where such a "cool core" of gas has been observed. Astronomers think these cool cores are important in understanding how quickly hot gas cools off in clusters, influencing the rate at which stars are born. This cooling rate could be slowed down by outbursts from a supermassive black hole in the center of the cluster. Apart from the cool core, the hot gas in the cluster is remarkably symmetrical and smooth. This is another piece of evidence that IDCS 1426 formed very rapidly in the early universe. Astronomers note that, despite the high mass and rapid evolution of this cluster, its existence does not pose a threat to the standard model of cosmology.

Hubble view of M83 -- the only galaxy known to host two potential "Eta twins." Its high rate of star formation increases the chances of finding massive stars that have recently undergone an Eta Carinae-like outburst. Bottom: Hubble data showing the locations of M83's Eta twins.

Bow shocks thought to mark the paths of massive, speeding stars are highlighted in this image from NASA's Wide-field Infrared Survey Explorer, or WISE. Cosmic bow shocks occur when massive stars zip through space, pushing material ahead of them in the same way that water piles up in front of a race boat. The stars also produce high-speed winds that smack into this compressed material. The end result is pile-up of heated material that glows in infrared light. In these images, infrared light has been assigned the colored red. Green shows wispy dust in the region and blue shows stars. The speeding stars thought to be creating the bow shocks can be seen at the center of each arc-shaped feature. This image actually consists of two bow shocks and two speeding stars. All the speeding stars are massive, ranging from about 8 to 30 times the mass of our sun.

Bow shocks thought to mark the paths of massive, speeding stars are highlighted in this image from NASA's Spitzer Space Telescope. Cosmic bow shocks occur when massive stars zip through space, pushing material ahead of them in the same way that water piles up in front of a race boat. The stars also produce high-speed winds that smack into this compressed material. The end result is pile-up of heated material that glows in infrared light. In these images, infrared light has been assigned the colored red. Green shows wispy dust in the region and blue shows stars. The speeding stars thought to be creating the bow shocks can be seen at the center of each arc-shaped feature. All the speeding stars are massive, ranging from about 8 to 30 times the mass of our sun.

Bow shocks thought to mark the paths of massive, speeding stars are highlighted in this image from NASA's Spitzer Space Telescope. Cosmic bow shocks occur when massive stars zip through space, pushing material ahead of them in the same way that water piles up in front of a race boat. The stars also produce high-speed winds that smack into this compressed material. The end result is pile-up of heated material that glows in infrared light. In these images, infrared light has been assigned the colored red. Green shows wispy dust in the region and blue shows stars. The speeding stars thought to be creating the bow shocks can be seen at the center of each arc-shaped feature. All the speeding stars are massive, ranging from about 8 to 30 times the mass of our sun.

This image shows an artist's impression of the 10 hot Jupiter exoplanets studied using the Hubble and Spitzer space telescopes. From top left to lower left, these planets are WASP-12b, WASP-6b, WASP-31b, WASP-39b, HD 189733b, HAT-P-12b, WASP-17b, WASP-19b, HAT-P-1b and HD 209458b. The colors of the planets are for illustration purposes only. There is little scientific data on color with the exception of HD 189733b, which became known as the "blue planet." The planets are also depicted with a variety of different cloud properties. The wind patterns shown on these 10 planets, which resemble the visible structures on Jupiter, are based on theoretical models. The illustrations are to scale with each other. HAT-P-12b, the smallest of these planets, is approximately the size of Jupiter, while WASP-17b, the largest one in the sample, is almost twice the size. The hottest planets within the sample are portrayed with a glowing night side. This effect is strongest on WASP-12b, the hottest exoplanet in the sample, but also visible on WASP-19b and WASP-17b. It is also known that several of the planets exhibit strong Rayleigh scattering. This effect causes the blue hue of the daytime sky and the reddening of the sun at sunset on Earth. It is also visible as a blue edge on the planets WASP-6b, HD 189733b, HAT-P-12b and HD 209458b.

This illustration shows a cool star, called W1906+40, marked by a raging storm near one of its poles. The storm is thought to be similar to the Great Red Spot on Jupiter. Scientists discovered it using NASA's Kepler and Spitzer space telescopes. The location of the storm is estimated to be near the north pole of the star based on computer models of the data. The telescopes cannot see the storm itself, but learned of its presence after observing how the star's light changes over time. The storm travels around with the star, making a full lap about every 9 hours. When it passes into a telescope's field of view, it causes light of particular infrared and visible wavelengths to dip in brightness. The storm has persisted for at least two years. Astronomers aren't sure why it has lasted so long. While planets are known to have cloudy storms, this is the best evidence yet for a star with the same type of storm. The star, W1906+40, belongs to a thermally cool class of objects called L-dwarfs. Some L-dwarfs are considered stars because they fuse atoms and generate light, as our sun does, while others, called brown dwarfs, are known as "failed stars" for their lack of atomic fusion. The L-dwarf W1906+40 is thought to be a star based on estimates of its age (the older the L-dwarf, the more likely it is a star). Its temperature is about 2,200 Kelvin (3,500 degrees Fahrenheit). That may sound scorching hot, but as far as stars go, it is relatively cool. Cool enough, in fact, for clouds to form in its atmosphere. W1906+40 is located 53 light-years away in the constellation Lyra.

This illustration shows a star behind a shattered comet. Observations of the star KIC 8462852 by NASA's Kepler and Spitzer space telescopes suggest that its unusual light signals are likely from dusty comet fragments, which blocked the light of the star as they passed in front of it in 2011 and 2013. The comets are thought to be traveling around the star in a very long, eccentric orbit.

The galaxy cluster called MOO J1142+1527 can be seen here as it existed when light left it 8.5 billion years ago. The red galaxies at the center of the image make up the heart of the galaxy cluster. This color image is constructed from multi-wavelength observations: Infrared observations from NASA's Spitzer Space Telescope are shown in red; near-infrared and visible light captured by the Gemini Observatory atop Mauna Kea in Hawaii is green and blue; and radio light from the Combined Array for Research in Millimeter-wave Astronomy (CARMA), near Owens Valley in California, is purple. In addition to galaxies, clusters also contain a reservoir of hot gas with temperatures in the tens of millions of degrees Celsius/Kelvin. CARMA was used to detect this gas, and to determine the mass of this cluster.

A massive cluster of galaxies, called SpARCS1049+56, can be seen in this multi-wavelength view from NASA's Hubble and Spitzer space telescopes. At the middle of the picture is the largest, central member of the family of galaxies (upper right red dot of central pair). Unlike other central galaxies in clusters, this one is bursting with the birth of new stars. Scientists say this star birth was triggered by a collision between a smaller galaxy and the giant, central galaxy. The smaller galaxy's wispy, shredded parts, called a tidal tail, can be seen coming out below the larger galaxy. Throughout this region are features called "beads on a string," which are areas where gas has clumped to form new stars. The right panel highlights the central galaxy and tidal tail. This type of "feeding" mechanism for galaxy clusters -- where gas from the merging of galaxies is converted to new stars -- is rare. The Hubble data in this image show infrared light with a wavelength of 1 micron in blue, and 1.6 microns in green. The Spitzer data show infrared light of 3.6 microns in red.

A massive cluster of galaxies, called SpARCS1049+56, can be seen in this multi-wavelength view from NASA's Hubble and Spitzer space telescopes. At the middle of the picture is the largest, central member of the family of galaxies (upper right red dot of central pair). Unlike other central galaxies in clusters, this one is bursting with the birth of new stars. Scientists say this star birth was triggered by a collision between a smaller galaxy and the giant, central galaxy. The smaller galaxy's wispy, shredded parts, called a tidal tail, can be seen coming out below the larger galaxy. Throughout this region are features called "beads on a string," which are areas where gas has clumped to form new stars. This type of "feeding" mechanism for galaxy clusters -- where gas from the merging of galaxies is converted to new stars -- is rare. The Hubble data in this image show infrared light with a wavelength of 1 micron in blue, and 1.6 microns in green. The Spitzer data show infrared light of 3.6 microns in red.

This montage displays an image released from each year of operation of NASA's Spitzer Space Telescope. Now celebrating it's 12th anniversary, Spitzer was first launched into space on August 25, 2003, from Cape Canaveral, Florida and is still going strong.

Scores of baby stars shrouded by dust are revealed in this infrared image of the star-forming region NGC 2174, as seen by NASAs Spitzer Space Telescope. Some of the clouds in the region resemble the face of a monkey in visible-light images, hence the nebula's nickname: the "Monkey Head." However, in infrared images such as this, the monkey disappears. That's because different clouds are highlighted in infrared and visible-light images. Found in the northern reaches of the constellation Orion, NGC 2174 is located around 6,400 light-years away. Columns of dust, slightly to the right of center in the image, are being carved out of the dust by radiation and stellar winds from the hottest young stars recently born in the area. Spitzers infrared view provides us with a preview of the next clusters of stars that will be born in the coming millennia. The reddish spots of light scattered through the darker filaments are infant stars swaddled by blankets of warm dust. The warm dust glows brightly at infrared wavelengths. Eventually, these stars will pop out of their dusty envelopes and their light will carve away at the dust clouds surrounding them. In this image, infrared wavelengths have been assigned visible colors we see with our eyes. Light with a wavelength of 3.5 microns is shown in blue, 8.0 microns is green, and 24 microns in red. The greens show the organic molecules in the dust clouds, illuminated by starlight. Reds are caused by the thermal radiation emitted from the very hottest areas of dust. Areas around the edges that were not observed by Spitzer have been filled in using infrared observations from NASAs Wide Field Infrared Survey Explorer, or WISE.

This plot captures the nearest known rocky exoplanet, dubbed HD 219134b, in the act of passing in front of its star. The data were obtained in infrared light using NASAs Spitzer Space Telescope. By carefully measuring the brightness of the star over several hours Spitzer easily detected the faint decrease in light that occurred when the planets disk blocked a tiny portion of the stars light. Even though the planet is 1.6 times the size of Earth, it still only accounts for less than a 0.04% reduction in the total light of the star during its transit. Since the host star is only 21 light years away it is quite bright and can even be seen with the naked eye, making it much easier to measure such a small change in brightness compared to other known transiting exoplanets that are much further away. The artwork accurately depicts the relative scale of the planet with respect to the star and the calculated path of the transit near the edge of the star's disk. Transiting planets are ideal targets for astronomers wanting to know more about their compositions and atmospheres. If molecules are present in the planet's atmosphere, they can absorb certain wavelengths of light, leaving imprints in the stars light during the transit. This type of technique also will be used in the future to investigate potentially habitable planets and search for signs of life. Now in its 11th year of operation, Spitzer has become an important tool for astronomers studying planets orbiting other stars.

This sky map shows the location of the star HD 219134 (circle), host to the nearest confirmed rocky planet found to date outside of our solar system. The star lies just off the "W" shape of the constellation Cassiopeia and can be seen with the naked eye in dark skies. It actually has multiple planets, none of which are habitable.

This artist's conception shows the silhouette of a rocky planet, dubbed HD 219134b, as it passes in front of its star. At 21 light-years away, the planet is the closest outside of our solar system that can be seen crossing, or transiting, its star -- a bonus for astronomers because transiting planets make ideal specimens for detailed studies of their atmospheres. It was discovered using the HARPS-North instrument on the Italian 3.6-meter National Galileo Telescope in the Canary Islands, and NASA's Spitzer Space Telescope. The planet, which is about 1.6 times the size of Earth, is also the nearest confirmed rocky planet outside our solar system. It orbits a star that is cooler and smaller than our sun, whipping closely around it in a mere three days. The proximity of the planet to the star means that it would be scorching hot and not habitable. Transiting planets are ideal targets for astronomers wanting to know more about planetary compositions and atmospheres. As a planet passes in front of its star, it causes the starlight to dim, and telescopes can measure this effect. If molecules are present in the planet's atmosphere, they can absorb certain wavelengths of light, leaving imprints in the starlight. This type of technique will be used in the future to investigate potentially habitable planets and search for signs of life.

This artist's rendition shows one possible appearance for the planet HD 219134b, the nearest rocky exoplanet found to date outside our solar system. The planet is 1.6 times the size of Earth, and whips around its star in just three days. Scientists predict that the scorching-hot planet -- known to be rocky through measurements of its mass and size -- would have a rocky, partially molten surface with geological activity, including possibly volcanoes.

This new composite image of stellar cluster NGC 1333 combines X-rays from NASAs Chandra X-ray Observatory (pink); infrared data from NASA's Spitzer Space Telescope (red); and optical data from the Digitized Sky Survey and the National Optical Astronomical Observatories Mayall 4-meter telescope on Kitt Peak near Tucson, Arizona.

This artist's concept shows a hypothetical "rejuvenated" planet -- a gas giant that has reclaimed its youthful infrared glow. NASA's Spitzer Space Telescope found tentative evidence for one such planet around a dead star, or white dwarf, called PG 0010+280 (depicted as white dot in illustration). When planets are young, they are warm and toasty due to internal heat left over from their formation. Planets cool over time -- until they are possibly rejuvenated. The theory goes that this Jupiter-like planet, which orbits far from its star, would accumulate some of the material sloughed off by its star as the star was dying. The material would cause the planet to swell in mass. As the material fell onto the planet, it would heat up due to friction and glow with infrared light. The final result would be an old planet, billions of years in age, radiating infrared light as it did in its youth. Spitzer detected an excess infrared light around the white dwarf PG 0010+280. Astronomers aren't sure where the light is coming from, but one possibility is a rejuvenated planet. Future observations may help solve the mystery. A Jupiter-like planet is about ten times the size of a white dwarf. White dwarfs are about the size of Earth, so one white dwarf would easily fit into the Great Red Spot on Jupiter!

This diagram illustrates how hypothetical helium atmospheres might form. These would be on planets about the mass of Neptune, or smaller, which orbit tightly to their stars, whipping around in just days. They are thought to have cores of water or rock, surrounded by thick atmospheres of gas. Radiation from their nearby stars would boil off hydrogen and helium, but because hydrogen is lighter, more hydrogen would escape. It's also possible that planetary bodies, such as asteroids, could impact the planet, sending hydrogen out into space. Over time, the atmospheres would become enriched in helium. With less hydrogen in the planets' atmospheres, the concentration of methane and water would go down. Both water and methane consist in part of hydrogen. Eventually, billions of years later (a "Gyr" equals one billion years), the abundances of the water and methane would be greatly reduced. Since hydrogen would not be abundant, the carbon would be forced to pair with oxygen, forming carbon monoxide. NASA's Spitzer Space Telescope observed a proposed helium planet, GJ 436b, with these traits: it lacks methane, and appears to contain carbon monoxide. Future observations are needed to detect helium itself in the atmospheres of these planets, and confirm this theory.