{"id":29088,"date":"2022-02-10T00:00:00","date_gmt":"2022-02-10T00:00:00","guid":{"rendered":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/?post_type=purple_issue&p=29088"},"modified":"2022-03-23T13:16:23","modified_gmt":"2022-03-23T13:16:23","slug":"growing-worlds","status":"publish","type":"post","link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/2022\/02\/10\/growing-worlds\/","title":{"rendered":"Growing Worlds"},"content":{"rendered":"\n
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<\/p>\n<\/div><\/div>\n\n

Growing Worlds<\/h2>\n\n

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<\/p>\n\n

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 \u2013 from worlds meeting their ends by tumbling into stars, to infant planets just beginning to form.<\/p>\n\n

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.<\/p>\n\n

\u201cI\u2019m interested in how planets form \u2013 not just extrasolar planets, but those in our Solar System too,\u201d says Bae. \u201cHistorically, we\u2019ve used theories, equations and simulations because observing planets while they\u2019re forming is extremely hard.\u201d Thanks to ALMA it\u2019s now possible \u2013 but still challenging \u2013 to view these planet-forming discs. <\/p>\n\n

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 \u2013 0.1 per cent of the star\u2019s 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\u2019t any close to Earth, so observing the planet-forming discs clearly is difficult.<\/p>\n\n

\"\"
LEFT: Astronomer Jaehan Bae creates a simulation of a planetary disc, in order to see what structure is created by the planet \u2013 in the lower section of the simulation
RIGHT: He can then make a comparison with a real observation of the young star SAO 206462 and its circumstellar disc<\/figcaption><\/figure><\/div>\n\n
Superficial details<\/strong><\/h5>\n\n

Nevertheless, many planet-forming discs have been found, raising the next challenge: to understand what\u2019s 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. \u201cBut it turns out they\u2019re not,\u201d says Bae. \u201cWe see plenty of different structures: rings and gaps, spirals <\/span>and vortices \u2013 all sorts of different things that suggest there might be a lot of planets.\u201d<\/p>\n\n

Even though ALMA can\u2019t observe the inner 10 astronomical units of any discs it sees clearly (1 AU is the distance between Earth and the Sun), there\u2019s 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\u2019s 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.<\/p>\n\n

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?<\/span><\/p>\n\n

\u201cAstronomers had thought planet formation happens a few million years after the star has formed,\u201d says Bae. \u201cBut we see these substructures in younger and younger circumstellar discs. We\u2019ve seen some that are very young, less than a million years old, which suggests planet formation could have already happened.\u201d<\/p>\n\n

\u201cIt\u2019s like trying to look for a firefly next to a lighthouse in Dublin, when you\u2019re in Edinburgh\u201d \u2013 Beth Biller <\/p><\/blockquote>\n\n

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An artist\u2019s impression of two gas giants orbiting the young star PDS 70, shows them accreting material from a surrounding disc<\/figcaption><\/figure><\/div>\n\n
Young planets, old stars<\/strong><\/h5>\n\n

Looking back from the other end of the timescale, however, makes the process far easier to work out. \u201cIn order to form a planet like Jupiter,\u201d says Bae, \u201cWe need gas, which means the planet has to form before the circumstellar disc disappears after around 10 million years.\u201d<\/p>\n\n

After this period, with no dusty disc to reveal what\u2019s 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.<\/span><\/p>\n\n

\u201cWe\u2019re 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\u2019re about the temperature of a candle flame,\u201d says Biller. \u201cIf you look in the infrared you can see them glowing.\u201d<\/p>\n\n

While stars also emit infrared light, they\u2019re dimmer at these wavelengths than in the visible part of the spectrum, so the star isn\u2019t overwhelmingly bright compared to the planet. But as the star is still 100,000 times brighter than a typical Jupiter-sized world, it\u2019s still quite a challenge. \u201cIt\u2019s like trying to look for a firefly next to a lighthouse in Dublin, when you\u2019re in Edinburgh,\u201d says Biller.<\/p>\n\n

To combat this brightness, astronomers use a coronagraph on their telescopes \u2013a filter that sits in front of the star, blocking out its glare and allowing them to see what\u2019s going on.<\/p>\n\n

\u201cYou need to be able to resolve something to within half an arcsecond,\u201d says Biller. \u201cSome telescopes such as Hubble have coronagraphs that block out the inner arcsecond \u2013 the entire real estate area in which you\u2019d be looking for planets.\u201d <\/p>\n\n

\"\"
The mirrors in both of the telescopes at the Keck Observatory feature adaptive optics to counteract the distortion caused by air currents<\/figcaption><\/figure><\/div>\n\n

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\u2019s 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\u2019s shape to compensate for distortion, but they\u2019re 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.<\/p>\n\n

Fortunately, NASA launched the James Webb Space Telescope on 25 December 2021 \u2013a 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 \u2013 an exercise primarily intended to calibrate the telescope for Webb\u2019s future exoplanet observations. When it\u2019s up to speed, JWST should be capable of detecting young planets down to the size of Uranus or Neptune.<\/span><\/p>\n\n

The real advantage of Webb, however, is that it isn\u2019t limited to seeing only wavelengths of infrared light that can pass through Earth\u2019s atmosphere; its position in deep space gives it access to infrared wavelengths denied to ground-based observatories.<\/span><\/p>\n\n

\u201cIf you take pictures over different wavelengths, you can look at the distribution of spectral energy for your object,\u201d says Biller. \u201cAnd spectroscopy is a powerful tool for understanding the atmospheres of these objects.\u201d<\/p>\n\n

Clues to the emergence of life<\/strong><\/h5>\n\n

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 \u2013 the building blocks of life. Discovering that these are present before a planet forms, and don\u2019t necessarily have to be created later, would have a huge impact on our theories of how life evolves.<\/p>\n\n

\u201cAnother thing we\u2019re looking at is following how planets move over the course of several years, tracing out their orbits around a young star,\u201d 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\u2019s gravity tugging on it. Sufficiently accurate measurements of the wobble reveal the masses of the orbiting planets.<\/span><\/p>\n\n

\"\"
By using the Keck Observatory and its telescopes equipped with coronagraphs to obscure the light from the star HR 8799, astronomers Jason Wang and Dr Christian Marois were able to capture these images of four planets orbiting the star over six years <\/figcaption><\/figure><\/div>\n\n
\"\"
The Extremely Large Telescope is currently under construction in northern Chile\u2019s Atacama Desert<\/figcaption><\/figure>\n\n

\u201cThe goal is to look at how the distribution changes as a function of age,\u201d says Biller. \u201cWe don\u2019t 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.\u201d But even when all these methods of observation are combined, we still don\u2019t have a complete picture. Current telescopes \u2013 even the JWST \u2013 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.<\/p>\n\n

\u201cWe hope to extend our methods to mature planets, which are colder, but that means moving to visual light,\u201d says Biller.<\/p>\n\n

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. \u201cThe Extremely Large Telescopes will also be able to probe regions closer to the star, down to about 1 AU,\u201d explains Bae.<\/p>\n\n

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.<\/p>\n\n

<\/p>\n\n

<\/div>
\n

How to grow a gas giant<\/span><\/h4>\n\n\n\n

Watching gas planets during their youth is key to understanding how they\u2019re born<\/strong><\/p>\n\n\n\n

\"\"
Precisely how gas giants like Jupiter form is still up for debate<\/span><\/figcaption><\/figure><\/div>\n\n\n\n

How clouds of dust transform into fully fledged gas giants is one of the key questions for astronomers researching young exoplanets.<\/p>\n\n\n\n

\u201cThere are two competing theories,\u201d says Jaehan Bae of the University of Florida.<\/p>\n\n\n\n

\u201cThe first is the \u2018bottom-up\u2019 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\u2019s the \u2018top-down\u2019 theory, where the disc becomes unstable due to its own gravity and collapses to form a planet. We don\u2019t know which is right, even for Jupiter.\u201d<\/p>\n\n\n\n

Both of these ideas have problems, however. In the \u2018bottom-up\u2019 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 \u2018top-down\u2019 model forms planets that are too massive to remain in stable orbits.<\/span><\/span><\/p>\n\n\n\n

Planets that are many times the mass of Jupiter fall into their stars far too readily.<\/p>\n\n\n\n

To delve deeper into the mystery, astronomers observe young planets while they\u2019re forming. Planets formed via the \u2018top-down\u2019 process are expected to be warmer than their \u2018bottom-up\u2019 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.<\/p>\n\n\n\n

\u201cIt\u2019s not a yes\/no question,\u201d Bae says. \u201cBoth\u00a0models can form giant planets, but they may work in different regions of the disc, or under different conditions.\u201d<\/span><\/p>\n<\/div><\/section>\n\n


Picturing planets<\/h4>\n\n

Even when taking direct images, it\u2019s not always clear cut if you\u2019ve caught one<\/strong><\/p>\n\n

\"\"
LEFT: The first direct image of an exoplanet, 2M1207b, captured in 2004
RIGHT: Hubble\u2019s image of planet candidate \u2018Fomalhaut b\u2019, and subsequent evidence (inset) of it fading from view<\/figcaption><\/figure>\n\n

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.<\/p>\n\n

More exoplanets have been directly imaged since then, but the difficulty in seeing such distant worlds means many of these observations remain controversial.<\/p>\n\n

\u201cIn terms of exoplanet detections we think are solid, there are around 20,\u201d says the University of Edinburgh\u2019s Professor Beth Biller. \u201cIt\u2019s something we fight about at conferences; the Exoplanet.eu<\/a> <\/strong>database lists 100 or so directly imaged exoplanets, but some are pretty suspect.\u201d<\/p>\n\n

One of the biggest controversies surrounds the planet \u2018discovered\u2019 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\u2019t take long for astronomers to find a bright object in the disc and announce they\u2019d found a planet, only for it to fade over the next few years. After much debate, the object was officially removed from NASA\u2019s exoplanet archive in 2020. Though it\u2019s still widely believed there\u2019s a planet around Fomalhaut, it\u2019s probably hidden among the dust of the disc. It just goes to show that seeing isn\u2019t always believing when it comes to direct imaging.<\/p>\n\n

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\"\"<\/figure><\/div>\n<\/div>\n\n\n\n
\n

Dr Ezzy Pearson is BBC Sky at Night Magazine<\/em>\u2019s news editor. Her book Robots in Space is available through History Press<\/p>\n<\/div>\n<\/div>\n\n

Photos: \u00a9 2022 W. M. KECK OBSERVATORY, ESO, NASA\/ESA AND P. KALAS (UNIVERSITY OF CALIFORNIA, BERKELEY AND SETI INSTITUTE), JASON WANG AND CHRISTIAN MAROIS X 2, ESO\/L. CAL\u00c7ADA<\/p>\n","protected":false},"excerpt":{"rendered":"

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