Imagine explorers from a distant star surveying our solar system. They would observe rocky planets tens of thousands of miles across, tiny dust particles around 600 times smaller than the period at the end of this sentence, and gas that’s formed into the largest planets.
If those explorers then turned their attention to distant stars where planets had yet to come into being, they would find those stars ringed with disks of material —the same kind of material they saw drifting through our already-formed solar system. The implication is clear — that material gave rise to planets. But how do those massive collections of gas and debris form into the orderly planetary bodies we see in our own solar system?
Understanding exoplanet formation is a complex task that utilizes NASA’s fleet of space telescopes. If we examine a newly forming star with an infrared telescope like the Spitzer Space Telescope, we can see gas, dust, and the young stars that remain hidden to an optical telescope like Hubble. Unfortunately, Spitzer cannot match the resolution of Hubble because it is a much smaller telescope. Its successor, NASA's James Webb Space Telescope, will also see infrared light, its giant mirror revealing the disks surrounding young stars with Hubble-like quality.
Webb will observe both gas and dust in exquisite detail around these young stars, studying micron-sized pebbles as they float in dense gas, colliding and merging gradually into ever-larger bodies. Our own solar system formed out of one of these “circumstellar” disks when the Sun was less than a million years old. But what did those formation steps look like?
• Do Jupiter-like gas giant planets form first as rocky cores and then accrete gas, or do they form in the earliest stages when a dense disk of gas collapses in on itself? Do planets like Earth form after the bigger objects have taken their share of the disk material, or are they somehow halted in the middle of their growth cycle?
• Are some planets engulfed by their host stars early on, or ejected by gravitational interactions with the disk or other planets later?
• How is water delivered to young, rocky planets? Is it via asteroids, comets, or another method?
• Could planets form that have no analogue here in the solar system?
Webb will address formation questions head-on, with its suite of spectrographs and imagers that can detect ices, gases, and dust particles in different parts of the planet-forming disk. In observing newly forming planetary systems, we gain insight into our own.
Hubble has taken actual images of exoplanets, but this is difficult because planets are tiny, faint, and very close to bright stars. The first images of exoplanets, called HR 8799 and Fomalhaut b, were taken in 2008 by the Keck and Gemini observatories and the Hubble Space Telescope, respectively. In order to see these dim objects next to very bright ones, we need a "high-contrast" camera, called a coronagraph, which can efficiently block out the light from the central star and let us see the much-dimmer planet orbiting around it. Webb, armed with its own coronagraph, will improve upon Hubble’s legacy of directly imaging giant exoplanets by observing in the infrared, where exoplanets emit most of their light.
The Webb telescope will also contain several spectrographs that will be trained on nearby exoplanets. Scientists will use these instruments to study the light of host stars as it passes through their planets’ atmospheres. Chemicals in the atmosphere cause color changes in the light that can be detected by spectrographs, allowing astronomers to glean information about the atmosphere’s composition. Furthermore, dust and gas can cloud atmospheres, leading to differences in brightness over time that can be mapped as exoplanetary “weather” by Webb.
The types and proportions of different gases can indicate processes occurring below the atmosphere level as well. Life shaped the atmosphere of the Earth, causing oxygen to increase and methane to decrease. With its ability to scrutinize the atmospheres of gas giant exoplanets, Webb will search for evidence of carbon dioxide, water vapor, and methane. In addition, Webb’s spectrographs will probe nearby rocky, Earth-sized planets for the chemical signs of a life-sustaining environment. Webb, and future missions, may finally provide an answer in the 21st century to the burning question: “Are we alone?”
For as long as we’ve gazed up at the stars, a question has burned in our minds: Is there life elsewhere in the universe? We still don’t know the answer. However, astronomers are using Hubble to get closer to it. They are studying planets outside the solar system to find out whether any of those worlds could support life.
This quest took a huge leap forward in the year 2000, when Hubble became the first telescope to directly detect an exoplanet’s atmosphere and survey its contents.
Astronomers used Hubble to study a planet 150 light-years away as it passed in front of its star, a yellowish Sun-like star called HD 209458. The planet, named HD 209458 b, was the first extrasolar planet known to make such “transits” across the face of its star. As the planet passed between its star and us, a small amount of light from the star was absorbed by the gas in the planet’s atmosphere, leaving chemical “fingerprints” in the star’s light. An instrument aboard Hubble called a spectrograph recorded the starlight and detected the signature of sodium that did not belong to the star. It was the mark of sodium gas in the atmosphere of the planet.
Since then, astronomers have discovered many more planets transiting their stars, and they’ve used Hubble to investigate some of those planets’ atmospheres, too. In the atmosphere of one planet called HD 189733 b, located 63 light-years away, Hubble detected methane. This was the first organic molecule identified in the atmosphere of a planet outside our solar system. Astronomers later used Hubble to pick up the signature of methane in the atmosphere of HD 209458 b as well. These exciting discoveries demonstrated that Hubble and future telescopes, such as the James Webb Space Telescope, could uncover the basic chemistry for life on other worlds.
So far, though, none of the planets whose atmospheres have been probed are likely to be habitable. Most are “hot Jupiters” — gaseous, Jupiter-like planets that have no solid surfaces and reside so close to their stars that their temperatures are unbearable. Astronomers have successfully directed Hubble to examine smaller planets, though. A Neptune-sized planet called HAT-P-11 b and a “super-Earth” (about 2.7 times bigger than our planet) called GJ 1214 b show evidence of water in their atmospheres. Still, both of these planets orbit very close to their stars and are extremely hot.
Conditions such as temperature and weather are important to know, too, when deciding whether a planet might be habitable. Hubble’s atmospheric observations are helping with this as well, revealing what the environment on these faraway worlds might be like.
For example, when Hubble first detected sodium in the atmosphere of HD 209458 b in 2000, astronomers actually saw less sodium than they expected. One possible explanation is that high-altitude clouds in the planet’s atmosphere might have been blocking some of the starlight and preventing us from observing more of the sodium. In several other planetary atmospheres, Hubble also detected weaker signals — or in some cases, no signal at all — from gases that astronomers expected to see. Such observations suggested that these planets may, too, have clouds or hazes in their atmospheres, obstructing the view.
Hubble’s observations of planets orbiting close to their stars have also revealed that some of their atmospheres are evaporating away. These planets are heated so much by their nearby stars that their atmospheres puff up and bleed into space, sometimes forming a comet-like tail behind the planet as it orbits. For HD 209458 b — one of these “evaporating” planets — Hubble detected gas streaming away at 22,000 miles (35,000 km) per hour. Astronomers estimate that at least 10,000 tons of hydrogen escapes from HD 209458 b’s atmosphere every second. Eventually, the bulk of the planet may disappear, leaving just its dense core behind.
For one hot Jupiter called WASP-43 b, astronomers used Hubble to create a temperature map for the planet. The planet is gravitationally locked to its star, so that it keeps one hemisphere always facing the star, just as the Moon keeps one face toward Earth. Hubble studied WASP-43 b when it was at different places in its orbit, and therefore had different sides facing toward us. Hubble revealed that the planet’s star-facing day side is a scorching 3,000 degrees Fahrenheit (1,600 degrees Celsius), while the night side was a slightly less brutal 1,000 degrees Fahrenheit (500 degrees Celsius). These temperature extremes would likely create winds that whip around the planet at the speed of sound.
Hubble remains at the forefront of analyzing extrasolar planet atmospheres because of its excellent stability and the sensitivity of its spectrographs. Sensing minuscule modifications to a star’s light is not impossible, but very difficult from the ground, where our planet’s atmosphere makes stars appear blurry. (It’s why stars appear to twinkle.) The crisp vision afforded by its position above Earth’s atmosphere, along with its ability to accurately point at an object for long periods of time, gives Hubble a better chance to see minute details in the light of a star so far away. Even for Hubble, though, detecting a planet’s atmosphere is a challenge, and astronomers are pushing the telescope to its limits to learn what they have about alien worlds trillions of miles away.
As we try to study smaller planets than the giants we’ve explored so far, it becomes even harder to measure the little bit of starlight passing through their atmospheres. If we aspire to analyze the atmospheres of Earth-sized planets, we’ll need even greater precision enabled by more powerful telescopes and advanced technologies, such as those provided by the James Webb Space Telescope. But someday, the kind of atmospheric observations Hubble is making today could reveal the chemical signatures of life on a far-off world. If life does exist among the stars, this could be how we find it.
When the Hubble Space Telescope launched in 1990, no one knew whether any planets existed outside our own solar system. Now, more than 5,000 may have been discovered, with more than 1,500 of those confirmed to be extrasolar planets.
However, extrasolar planets have always been — and still are — difficult to find. Some can be a billion times dimmer than their brilliant parent stars, making them impossible to see directly. Astronomers have had to devise clever and highly precise techniques to uncover extrasolar planets. Finding a new extrasolar planet often requires a telescope to stare at a star for days, months, or years in order to observe the effects of the planet on its parent star or local environment.
NASA’s Kepler Space Telescope is a dedicated planet hunter; its primary mission is to perform an extrasolar-planet census of our galaxy. Supporting that effort, Hubble has used its unique capabilities for planet detection and to pioneer extrasolar planet-finding techniques beyond the transit method used by Kepler.
Among Hubble’s unique capabilities is its truly exquisite resolution, which allows Hubble to see small details in its celestial targets. Hubble’s fine resolution stems from its relatively large primary mirror, its placement above the blurring effects of Earth’s atmosphere, and in its stability. Hubble’s stability enables the telescope to point with extreme precision and to remain fixed on a target over a long period of time — eliminating the tiny wiggles in motion that would cause its images to blur.
It is not just Hubble’s fine resolution that allows for the detection of extrasolar planets. Hubble’s instruments are very sensitive and provide a means for astronomers to do high-contrast imaging — or the ability to see a very faint signal (such as an extrasolar planet) near a very bright source (a star). The combination of stability, placement above Earth’s atmosphere, and state-of-the-art instruments means that Hubble remains one of the most capable planet-finding observatories in use today. In fact, astronomers are pioneering new methods to find extrasolar planets with Hubble.
Astronomers are making “pre-discoveries” of extrasolar planets with archival Hubble data. Coronagraphs — telescope or instrument attachments that block the light from a star — are common on modern telescopes. When a coronagraph is used to block the light from a star, fainter objects near the star can often be detected. Hubble’s coronagraphs, in tandem with the telescope’s superb resolution, have allowed astronomers to find new extrasolar planets.
By re-analyzing archived coronagraphic Hubble images taken in 1998, astronomers spotted three planets around a star called HR 8799. These planets were discovered in infrared images taken with large ground-based telescopes in 2007 and 2008. Astronomers did not originally see the planets in Hubble’s 1998 images because the star’s light was too bright. Even after the star’s light was removed from the images by the coronagraph, scattered light from the star overwhelmed the faint light from the planets. Since then, though, astronomers have developed new software processing techniques to suppress the starlight further, allowing the planets to stand out. These older Hubble observations are invaluable for tracking the planets’ motions and estimating their orbits. Because the planets’ orbits are 100 to 400 years long, having images that span a decade helps astronomers to measure the planets’ motions during their slow journeys around their stars. Combined with Hubble’s vast archive containing 25 years of images and data, these new analysis techniques might assist in more “pre-discoveries” — or perhaps original discoveries — of extrasolar planets in Hubble observations.
Astronomers are finding signs of extrasolar planets via Hubble observations of disks around stars. Astronomers had long suspected that a planet orbits the bright star Fomalhaut after observing suspicious features in the debris disk that surrounds the star. The disk was not centered on Fomalhaut, and it had a sharp inner edge that seemed to have been gravitationally groomed by a planet orbiting between the star and the disk. Any planet that existed there eluded detection, however, until Hubble turned its attention to Fomalhaut. Hubble observed a faint source of light — a billion times dimmer than Fomalhaut — moving in a gentle arc around the star, near the disk’s inner edge. Hubble has continued following the planet’s motion for several years since its first observations in 2004. This has allowed astronomers to calculate that the planet has a 2,000-year-long, highly elliptical orbit around the star.
Hubble has been at the forefront of observing disks to better understand how young stars and planets form and evolve. In addition to Fomalhaut, astronomers using Hubble have inferred the possible existence of extrasolar planets around several other stars with disks, including TW Hydrae, HD 141569, and Beta Pictoris.
Hubble is rewriting the record books on extrasolar-planet discoveries. Observing extrasolar planets directly is incredibly difficult. Of the nearly 2,000 confirmed extrasolar planets discovered, only a handful have been observed directly, and most of those in infrared light where they appear relatively brighter. Thanks again to Hubble’s stability and its ability to take high-contrast images, Hubble’s images of Fomalhaut’s planet were the first to capture an extrasolar planet in visible light.
Hubble has found some of the most distant extrasolar planets ever discovered. For one week in 2004, a survey called the Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS) used Hubble to measure the light from 180,000 stars near the center of our Milky Way Galaxy, roughly 26,000 light-years away. Hubble recorded a periodic dimming in 16 of those stars. Such dimming events could be caused by a planet passing in front of a star as the planet orbits it, regularly blocking out some of the star’s light. By measuring shifts in each star’s position (detected as shifting lines in the stars’ spectra), astronomers using ground-based telescopes confirmed that planets do orbit two of those 16 stars. The other 14 stars that were seen to dim periodically were too faint to conduct such measurements on reliably. However, future telescopes might be able to re-investigate them and reveal whether they do indeed have orbiting planets.
Hubble continues to make significant contributions to the search for other worlds. It has discovered extrasolar planets as well as helped confirm the existence of extrasolar planets found by other telescopes. Hubble’s longevity, stability, and instrumental sensitivity make it a unique member of NASA’s planet-hunting telescopes.
Astronomers have confirmed the existence of more than 1,500 planets orbiting other stars in our galaxy. Investigations of these extrasolar planets is a burgeoning field in astronomy, and Hubble has made important contributions.
The Hubble Space Telescope's exquisite resolution and sensitivity can enable studies of the debris disks around young stars that give rise to planets. Such disks are difficult to observe, as they surround a star that is typically 100,000 times brighter than the disk. Hubble's ability to perform high-contrast imaging, in which the overwhelming light from the star is blocked, has provided observations of about two dozen of these disks. Though the disks only reflect visible light from the star, they glow in infrared light. Improvements to Hubble's infrared instruments have made possible new and more detailed observations.
Astronomers are using both old and new Hubble observations to discover disks forming around other stars.
- Finding Disks in Archived Data: Applying new image-processing techniques, astronomers have been able to tease out images of disks hidden away in Hubble infrared data taken years ago. Such discoveries underscore the importance of archiving astronomical observations for future astronomers.
- New Survey of Disks: Astronomers have completed the largest and most sensitive visible-light imaging survey of dusty debris disks around other stars. These dusty disks, likely created by collisions between leftover objects from planet formation, were imaged around stars as young as 10 million years old and as mature as more than 1 billion years old.
These dust-disk searches have started to reveal surprising characteristics of planet-forming disks, and no two look the same. These are not uniform flat disks; they are three-dimensional shapes that include small-scale structures. For instance, irregularities observed in one ring-like system resemble the ejection of a huge spray of debris into the outer part of the system from the recent collision of two bodies. The challenge for astronomers is to understand the influences on these systems, including the role of stellar winds, interactions with clouds of interstellar material, the mass and age of the parent star, and the quantity of heavier elements required to build planets.
From this small sample, the most important message is one of diversity. Because planets form within these disks, the shapes of the debris disks should reflect the architectures of the forming planetary systems. The Hubble results are consistent with extrasolar planet observations, where planets are found arranged in orbits that are markedly different than those seen in our solar system.
These new disk surveys also yield insight into how our solar system formed and developed. In particular, the suspected planet collision mentioned above may be similar to how the Earth-Moon system or the Pluto-Charon system formed over 4 billion years ago. In those cases, collisions between planet-sized bodies cast debris that then coalesced into companion moons.
This sort of survey is the first of many more that will become possible with the launch of the James Webb Space Telescope in the next decade. Each of these systems may be observed in more detail in the infrared with JWST, potentially revealing evidence of newly formed planets.
Watch throughout the year for more articles on Hubble's 25 years of discovery.