Recognizing Worlds Beyond Our Sun

Hubble’s unique capabilities allow it to explore planetary systems around other stars. 

Artist's impression of the ten hot jupiter exoplanets studied by astronomer david sing and his colleagues

When Hubble launched in 1990, there were no confirmed planets outside of our solar system. Scientists have since established the existence of more than 5,000 extrasolar planets, most of them discovered by NASA’s Kepler and TESS space observatories and by ground-based telescopes. Hubble, however, has also made some unique contributions to the planet hunt.

Astronomers use Hubble to take the first measurements of the atmospheric composition of extrasolar planets. Its observations have identified atmospheres that contain sodium, oxygen, carbon, hydrogen, carbon dioxide, methane, helium, and water vapor. Most of the planetary bodies studied to date are too hot for life as we know it. But Hubble observations demonstrate that we can detect and measure the basic organic components for life on planets orbiting other stars.

Artist's concept of TRAPPIST-1 planetary system. At left is a sliver of the star TRAPP!IS-1. To its are artists concepts of each of the seven known planets.
This artist's concept shows what the TRAPPIST-1 planetary system may look like, based on available data about the planets' diameters, masses and distances from the host star, as of February 2018.

Hubble observed the first known system of seven Earth-size planets around a single star. The star, TRAPPIST-1, is as an ultra-cool dwarf that would allow liquid water to survive on planets orbiting close to the star, closer than is possible on planets in our own solar system. All seven of the TRAPPIST-1 planets have tight orbits. They orbit closer to their star than Mercury is to our Sun, and are also orbiting very close to each other. If you stood on one of these planets, you could potentially see geological features or clouds on your neighboring worlds. Hubble observations revealed that at least three of the exoplanets appear to not have puffy, hydrogen-rich atmospheres similar to gaseous planets like Neptune. This favors more compact atmospheres like those found on Earth, Venus, and Mars.

Top two-thirds of the image holds a bright-light-yellow star at center. Around the star is the oval orbit of the planet. Points along the orbit indicate the planet's location and temperature measurements. The bottom third of the image holds four spheres, close-ups of the individual temperature maps.
This is a temperature map of exoplanet WASP-43b. The gas giant planet orbits very close to its parent star with a period of 19.5 hours. Because the planet keeps one side facing its star, there are huge temperature extremes between the day and night sides. The white-colored region on the daytime side is 2,800 degrees Fahrenheit. The nighttime-side temperatures drop below 1,000 degrees Fahrenheit. This steep gradient is in stark contrast to the predominantly uniform temperatures of the solar system's giant planets. Infrared observations with the Hubble Space Telescope measured how temperatures change with both altitude and longitude on the planet.
NASA, ESA, and K. Stevenson, L. Kreidberg, and J. Bean (University of Chicago)

In another case, astronomers had sufficient data to make a detailed global map of an exoplanet (WASP-43b) showing the temperatures at different layers in its atmosphere, and the amount and distribution of its water vapor.

Artist's impression of the ten hot jupiter exoplanets studied by astronomer david sing and his colleagues
This image shows an artist’s impression of the ten hot Jupiter exoplanets. From top left to 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 images are to scale with each other. HAT-P-12b, the smallest of them, is approximately the size of Jupiter, while WASP-17b, the largest planet in the sample, is almost twice the size. The planets are also depicted with a variety of different cloud properties. There is almost no information about the colors of the planets available, with the exception of HD 189733b, which became known as the blue planet (heic1312). 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. The wind patterns shown on these ten planets, which resemble the visible structures on Jupiter, are based on theoretical models.
ESA/Hubble & NASA

Hubble’s observations of exoplanets have also revealed some truly strange worlds; including one that absorbs nearly all of the light that reaches it. Most planets reflect some of their host star’s light back into space, but this oddity (WASP-12b) is as dark as fresh asphalt. Another is an extremely hot, football-shaped world that is streaming away heavy metal iron and magnesium gas (WASP-121b). Hubble observations also confirmed that three super-puffy planets in the Kepler 51 system have extremely low densities. While these planets appear to be as big and bulky as Jupiter, they are actually one hundred times less massive, leading researchers to nickname them ‘cotton candy’ planets.

size comparison of planets in our solar system with planets in Kepler 51 system
This illustration depicts the three giant planets orbiting the Sun-like star Kepler 51 as compared to some of the planets in our solar system. These planets are all roughly the size of Jupiter but a very tiny fraction of its mass. NASA's Kepler space telescope detected the shadows of these planets in 2012–2014 as they passed in front of their star. There is no direct imaging. Therefore, the colors of the Kepler 51 planets in this illustration are imaginary.
NASA, ESA, and L. Hustak and J. Olmsted (STScI)

With its ultraviolet-light capabilities, Hubble uncovered one planet slowly accumulating matter (PDS 70b) and two others (GJ 436b, GJ3470b) that are shrinking as their atmospheres bleed off hydrogen. The growing planet is a massive Jupiter-sized world weighing between 4 to 17 times as much as Jupiter. The shrinking, warm, Neptune-sized planets are evaporating hydrogen due to extreme radiation from their respective stars. These shrinking worlds could explain the existence of so-called hot super-Earths, which may result from a similar process that stripped away the Neptune-sized planets’ atmospheres exposing their rocky cores.

Blue background. Center of image is a disk blocking the light of a star. Below and just to the left of the disk, at about seven o'clock, is a bright white point. This is PDS 70b.
Hubble’s sensitivity to ultraviolet light reveals radiation from super-heated gas falling onto a world called PDS 70b. The glare of the star was blocked, allowing Hubble to directly observe PDS 70b accumulating mass. Located some 370 light-years from Earth, the planet is about five times the mass of Jupiter and growing at a snail’s pace. Researchers found that the planet is growing so slowly that if the rate remains steady for another million years, its bulk will increase by only about 1/100th of Jupiter’s mass.
NASA, ESA, and Y. Zhou (McDonald Observatory/University of Texas)

Using another technique, called gravitational microlensing, Hubble astronomers also confirmed the existence of a Saturn-mass planet orbiting two small, faint stars in a tight orbit around each other. The system, called OGLE-2007-BLG-349, is 8,000 light-years away toward the center of our galaxy. Gravitational microlensing occurs when the gravity of a moving foreground star bends and amplifies the light of a background star that temporarily aligns with it along our line of sight. Details in the character of the brightening reveals clues about nature of the foreground star and any planets it may have.

artist concept of gas giant planet circling a pair of red dwarf stars in the system OGLE-2007-BLG-349
This artist's illustration shows a gas giant planet circling a pair of red dwarf stars in the system OGLE-2007-BLG-349, located 8,000 light-years away. The Saturn-mass planet orbits roughly 300 million miles from the stellar duo. The two red dwarf stars are 7 million miles apart.
NASA, ESA, and G. Bacon (STScI)

In 2004, Hubble began regularly observing what astronomers thought might be an extrasolar planet. Astronomers studied the suspected planet for 16 years, tracking its movements around the bright nearby star Fomalhaut, located about 25 light-years away. However, something strange was happening as the planet appeared to dim with each successive observation. Then, in 2020, it seemed to vanish completely. Further research revealed that the suspected planet may actually be a vast, expanding cloud of dust produced in a collision between two large bodies orbiting the nearby star Fomalhaut.  

Hubble observations and data simulation of Fomalhaut star system
The color-tinted Hubble image on the left is a vast ring of icy debris encircling the star Fomalhaut. The star is so bright that a black occulting disk blocked out its glare so that Hubble could photograph the dust ring. In 2008, astronomers announced a potential exoplanet and released the first direct image of the suspected planet orbiting far from the star. However, by 2014, the planetary object faded below Hubble's detection. The best interpretation is that the object wasn't a fully formed planet at all, but an expanding cloud of dust from a collision between two minor bodies, each about 125 miles across. The cloud, made of very fine dust particles, has an estimated size of more than 200 million miles across. Astronomers estimate that smashups like this happen around Fomalhaut once every 200,000 years; so Hubble was looking at the right place at the right time to capture the transient event. The diagram at right is based on a simulation of the expanding and fading cloud. 
NASA, ESA, and A. Gáspár and G. Rieke (University of Arizona)
animation of Hubble Fomalhaut observations
This animation shows Fomalhaut b’s movement over time.
NASA, ESA, P. Kalas (University of California, Berkeley) and G. Bacon (STScI)

Hubble E-Book

Hubble Focus: Strange New Worlds

Hubble Focus is a series of e-books that dive deeper into specific topics in astronomy that have been forever changed by Hubble’s explorations. "Hubble Focus: Strange New Worlds" is the fourth book in the series, highlighting the mission’s recent discoveries about worlds outside our solar system, known as exoplanets.

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Hubble Focus - Strange New Worlds e-book cover
E-book cover for Hubble Focus: Strange New Worlds

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