Growing Worlds
The journey from dust cloud to planet has long been shrouded in mystery, but astronomers are now getting a clear view of how young exoplanets grow, says Ezzy Pearson
An explosion of exoplanet discoveries has swept through astronomy in the last 30 years. Prior to 1989, humanity only knew of the planets in our own Solar System, but today the number of confirmed extrasolar planets is nearing 5,000 thanks to space telescopes like Kepler and the Transiting Exoplanet Survey Satellite (TESS) scanning the stars for other worlds. The more exoplanets that are found, the greater the diversity of worlds we know about – from worlds meeting their ends by tumbling into stars, to infant planets just beginning to form.
The study of these young planets and the circumstellar discs of dust and gas from which they emerge has blossomed as a field in recent years. Observatories such as the Atacama Large Millimeter/submillimeter Array (ALMA) have allowed astronomers such as Jaehan Bae from the University of Florida to examine the discs in more detail than ever before.
“I’m interested in how planets form – not just extrasolar planets, but those in our Solar System too,” says Bae. “Historically, we’ve used theories, equations and simulations because observing planets while they’re forming is extremely hard.” Thanks to ALMA it’s now possible – but still challenging – to view these planet-forming discs.
First though, you have to find one. Although stars like the Sun live for around 10 billion years, their discs last for only five to 10 million years – 0.1 per cent of the star’s lifespan. The best places to spot them are in the regions around stellar nurseries, where new stars are forming. Unfortunately, these are dust-rich environments and there aren’t any close to Earth, so observing the planet-forming discs clearly is difficult.
RIGHT: He can then make a comparison with a real observation of the young star SAO 206462 and its circumstellar disc
Superficial details
Nevertheless, many planet-forming discs have been found, raising the next challenge: to understand what’s happening in them. Prior to their first observations, astronomers thought this might be a tricky prospect as they expected the discs to be smooth, featureless and, frankly, rather boring. “But it turns out they’re not,” says Bae. “We see plenty of different structures: rings and gaps, spirals and vortices – all sorts of different things that suggest there might be a lot of planets.”
Even though ALMA can’t observe the inner 10 astronomical units of any discs it sees clearly (1 AU is the distance between Earth and the Sun), there’s no shortage of features to spot in their outer regions. Some of these are easy to explain. As they form, planets often clear out great swathes of dust, leaving dark lanes and sharp edges to betray their presence. Sometimes it’s even possible to directly image nascent planets as bright points of light. In 2021, observations of the planet PDS 70c revealed that it had surrounded itself with its own disc, from which it could one day form a family of moons.
But not all features are clear, particularly in younger discs where the planets are still forming. Here, all there is to go on are the patterns in the dust, created by the movements of hidden planets. To connect these obvious features with the physical process that cause them, Bae is creating computer simulations of the discs. By comparing these with their real-life counterparts, he can begin to link what we see with the physical processes they indicate. His studies will help answer one of the key questions in this field: when do planets begin to form?
“Astronomers had thought planet formation happens a few million years after the star has formed,” says Bae. “But we see these substructures in younger and younger circumstellar discs. We’ve seen some that are very young, less than a million years old, which suggests planet formation could have already happened.”
“It’s like trying to look for a firefly next to a lighthouse in Dublin, when you’re in Edinburgh” – Beth Biller
Young planets, old stars
Looking back from the other end of the timescale, however, makes the process far easier to work out. “In order to form a planet like Jupiter,” says Bae, “We need gas, which means the planet has to form before the circumstellar disc disappears after around 10 million years.”
After this period, with no dusty disc to reveal what’s happening, astronomers must rely on other forms of observation. Fortunately, young exoplanets are one of the only types of planet that astronomers such as Professsor Beth Biller from the University of Edinburgh are able to capture directly the light from, in the form of infrared radiation.
“We’re looking at stars [that are] about five to a hundred million years old. Young planets, those closer to formation, are hotter [than stars that old]: at their cloud tops they’re about the temperature of a candle flame,” says Biller. “If you look in the infrared you can see them glowing.”
While stars also emit infrared light, they’re dimmer at these wavelengths than in the visible part of the spectrum, so the star isn’t overwhelmingly bright compared to the planet. But as the star is still 100,000 times brighter than a typical Jupiter-sized world, it’s still quite a challenge. “It’s like trying to look for a firefly next to a lighthouse in Dublin, when you’re in Edinburgh,” says Biller.
To combat this brightness, astronomers use a coronagraph on their telescopes –a filter that sits in front of the star, blocking out its glare and allowing them to see what’s going on.
“You need to be able to resolve something to within half an arcsecond,” says Biller. “Some telescopes such as Hubble have coronagraphs that block out the inner arcsecond – the entire real estate area in which you’d be looking for planets.”
Some ground-based observatories, such as the Very Large Telescope (VLT) in Chile and the Keck Telescopes in Hawaii, have been updated with modern, small-angle coronagraphs, improving their ability to make these observations. But they still face another problem: Earth’s atmosphere. The air in the atmosphere above the telescope is constantly moving, which causes the image to blur, and also absorbs much of the infrared radiation passing through it. Both VLT and Keck compensate for the unsteady air with adaptive optics, adjusting the mirror’s shape to compensate for distortion, but they’re still limited to observing in the narrow range of infrared wavelengths that can pass through our atmosphere. The only way to get around that issue is to get above the atmosphere.
Fortunately, NASA launched the James Webb Space Telescope on 25 December 2021 –a 6.5m-wide telescope designed to observe in the infrared part of the spectrum and which has a coronagraph. Once the telescope is in full scientific order in mid-2022, Biller will lead a team that will use Webb to directly image its first exoplanets – an exercise primarily intended to calibrate the telescope for Webb’s future exoplanet observations. When it’s up to speed, JWST should be capable of detecting young planets down to the size of Uranus or Neptune.
The real advantage of Webb, however, is that it isn’t limited to seeing only wavelengths of infrared light that can pass through Earth’s atmosphere; its position in deep space gives it access to infrared wavelengths denied to ground-based observatories.
“If you take pictures over different wavelengths, you can look at the distribution of spectral energy for your object,” says Biller. “And spectroscopy is a powerful tool for understanding the atmospheres of these objects.”
Clues to the emergence of life
As well as helping to give a better idea of what planets outside the Solar System are like, these types of observations will also help to solve other mysteries, such as how life evolved on Earth and whether it could have evolved elsewhere. ALMA has already shown that several planet-forming discs contain simple organic molecules – the building blocks of life. Discovering that these are present before a planet forms, and don’t necessarily have to be created later, would have a huge impact on our theories of how life evolves.
“Another thing we’re looking at is following how planets move over the course of several years, tracing out their orbits around a young star,” Biller says. As well as directly observing planets and the structure of planetary discs, astronomers use another well-established method to find and monitor exoplanets: radial velocity measurements. This method relies on closely monitoring a star in order to detect wobbles caused by an orbiting planet’s gravity tugging on it. Sufficiently accurate measurements of the wobble reveal the masses of the orbiting planets.
“The goal is to look at how the distribution changes as a function of age,” says Biller. “We don’t expect these to be identical to what mature planets in our Solar System look like. We expect them to still be moving around in that state.” But even when all these methods of observation are combined, we still don’t have a complete picture. Current telescopes – even the JWST – can only find gas giants that are far from their stars, beyond around 10 AU, which rules out the class of planets most eagerly sought after by exoplanet researchers: Earth-like planets.
“We hope to extend our methods to mature planets, which are colder, but that means moving to visual light,” says Biller.
Doing so will require even more advanced telescopes, such as the Large Ultraviolet Optical Infrared Telescope (LUVOIR), a multiwavelength space telescope with a potential diameter of over 15m, currently being proposed by NASA as a successor to Webb. But while LUVOIR is still on the drawing board, there is a new generation of huge ground-based observatories with mirrors over 30m in diameter, known as Extremely Large Telescopes (ELTs), already under construction and expected to come online in the 2030s. “The Extremely Large Telescopes will also be able to probe regions closer to the star, down to about 1 AU,” explains Bae.
As our knowledge of mature exoplanets has grown, their more youthful counterparts have remained hidden beyond the limits of our observing capabilities. But over the coming years, as more advanced telescopes come online, these worlds will begin to emerge from the shadows and reveal the history of planetary systems like our own.
How to grow a gas giant
Watching gas planets during their youth is key to understanding how they’re born
How clouds of dust transform into fully fledged gas giants is one of the key questions for astronomers researching young exoplanets.
“There are two competing theories,” says Jaehan Bae of the University of Florida.
“The first is the ‘bottom-up’ process, where you initially form a small, rocky core that becomes more massive. It starts to collect gas from the disc and forms a gas giant like Jupiter. Then there’s the ‘top-down’ theory, where the disc becomes unstable due to its own gravity and collapses to form a planet. We don’t know which is right, even for Jupiter.”
Both of these ideas have problems, however. In the ‘bottom-up’ model, regions of the disc far from the star are too cold for planets like Neptune to form within the 10 million-year window before the disc disappears. On the other hand, the ‘top-down’ model forms planets that are too massive to remain in stable orbits.
Planets that are many times the mass of Jupiter fall into their stars far too readily.
To delve deeper into the mystery, astronomers observe young planets while they’re forming. Planets formed via the ‘top-down’ process are expected to be warmer than their ‘bottom-up’ counterparts, so mapping out the mass and temperature of forming planets could yield vital clues. Meanwhile, tracking how planets are distributed in the disc at different points in their lives will reveal more about their patterns of migration.
“It’s not a yes/no question,” Bae says. “Both models can form giant planets, but they may work in different regions of the disc, or under different conditions.”
Picturing planets
Even when taking direct images, it’s not always clear cut if you’ve caught one
RIGHT: Hubble’s image of planet candidate ‘Fomalhaut b’, and subsequent evidence (inset) of it fading from view
The first direct image of an exoplanet was captured in 2004 by the Very Large Telescope. The planet, named 2M1207b, is around five times the mass of Jupiter, orbiting 42 AU from a brown dwarf star in the TW Hydrae association of stars, around 200 lightyears from Earth and a youthful 8 million years old.
More exoplanets have been directly imaged since then, but the difficulty in seeing such distant worlds means many of these observations remain controversial.
“In terms of exoplanet detections we think are solid, there are around 20,” says the University of Edinburgh’s Professor Beth Biller. “It’s something we fight about at conferences; the Exoplanet.eu database lists 100 or so directly imaged exoplanets, but some are pretty suspect.”
One of the biggest controversies surrounds the planet ‘discovered’ orbiting the star Fomalhaut in 2008. The star is clearly surrounded by a dust disc with a sharp inner edge, suggesting a planet clearing out the dust. It didn’t take long for astronomers to find a bright object in the disc and announce they’d found a planet, only for it to fade over the next few years. After much debate, the object was officially removed from NASA’s exoplanet archive in 2020. Though it’s still widely believed there’s a planet around Fomalhaut, it’s probably hidden among the dust of the disc. It just goes to show that seeing isn’t always believing when it comes to direct imaging.
Dr Ezzy Pearson is BBC Sky at Night Magazine’s news editor. Her book Robots in Space is available through History Press