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<\/p>\nNew Angles on Planet Formation
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Planet formation is a fascinating subtopic of the exoplanet hunt, and it may just have produced the first exoplanet detection in data that go back as far back as 1981, though the event in question has never been confirmed as being caused by a planet. I learned this through a paper sent me recently by Jean Schneider (Observatoire de Paris), who along with colleague Danielle Briot wrote about the early days of transit searches in a chapter for the Handbook of Exoplanets<\/em> (Springer, 2018).<\/p>\n I want to dig deeper into that chapter in a later post, but for now, I note that the planet Beta Pictoris b, discovered in 2008 and orbiting an infant star 63 light years from Earth, may have transited in 1981, according to subsequent papers on the matter. The debris disk around the primary has long fascinated astronomers and it has been investigated for the possible presence of comet-like bodies and subjected to direct imaging searches, which revealed Beta Pictoris b and confirmed it in 2009. But the 1981 data show light variations that could be interpreted as a transit.<\/p>\n Image:<\/strong> Various planet formation processes, including exocomets and other planetesimals, around Beta Pictoris, a very young type A V star. Credit: NASA\/FUSE\/Lynette Cook. <\/p>\n Testing the matter involved a nano-satellite called PicSat designed by astronomers at the Paris-Meudon observatory to look for a possible 2018 transit. Unfortunately, the mission failed when communications were lost. As Schneider and Briot write:<\/p>\n If the transit is confirmed in the near future, we could obtain better observations of the following transit in 2053, when the period will be known more accurately and when we could use very large and extraordinary outstanding future instruments. So, will Beta Pictoris b win the title of the first detected exoplanet?<\/p>\n<\/blockquote>\n The massive debris disk at Beta Pictoris is asymmetric (likely the result of perturbations by another star), and seen edge-on from Earth. But the beauty of the circumstellar disks out of which planets form is that if we peer into planet-forming regions, we can find them oriented in all kinds of ways, as we learn in a new series of images from the European Southern Observatory\u2019s Very Large Telescope. They\u2019re part of a survey of how planetary systems form, and they mark a transition between the intense study of individual star systems to a broad swath of planets and stars. As Christian Ginski (University of Galway, Ireland) puts it, \u201cWe\u2019ve gone from the intense study of individual star systems to this huge overview of entire star-forming regions.\u201d<\/p>\n Let\u2019s home in on a single system to begin with to ponder how much we can learn from these disks.<\/p>\n Image:<\/strong> This composite image shows the MWC 758 planet-forming disc, located about 500 light-years away in the Taurus region, as seen with two different facilities. The yellow colour represents infrared observations obtained with the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument on ESO\u2019s Very Large Telescope (VLT). The blue regions on the other hand correspond to observations performed with the Atacama Large Millimeter\/submillimeter Array (ALMA), in which ESO is a partner. These facilities allow astronomers to map how dust is distributed around this and other stars in different but complementary ways. SPHERE captures light from the host star that has been scattered by the dust around it, whereas ALMA registers radiation directly emitted by the dust itself. These observations combined help astronomers understand how planets may form in the dusty discs surrounding young stars. Credit: ESO\/A. Garufi et al.; R. Dong et al.; ALMA (ESO\/NAOJ\/NRAO).<\/p>\n The new ESO imagery takes in numerous young stars like this one. What leaps out here is the wide range of formation scenarios. Working with the star-forming regions at Orion, Taurus and Chamaeleon I, an international team of astronomers investigated 86 stars using SPHERE, the Spectro-Polarimetric High-contrast Exoplanet REsearch instrument mounted on the VLT. The quality of the imagery is attributable to SPHERE\u2019s adaptive optics, which allows disks to be imaged around stars down to about half the Sun\u2019s mass while correcting for the distortions produced by looking through the Earth\u2019s atmosphere. The use of ALMA, the Atacama Large Millimeter\/submillimeter Array, helped quantify the amount of dust in these systems.<\/p>\n Image:<\/strong> This research brings together observations of more than 80 young stars that might have planets forming around them in spectacular discs. This small selection from the survey shows 10 discs from the three regions of our galaxy observed in the papers. V351 Ori and V1012 Ori are located in the most distant of the three regions, the gas-rich cloud of Orion, some 1600 light-years from Earth. DG Tau, T Tau, HP Tau, MWC758 and GM Aur are located in the Taurus region, while HD 97048, WW Cha and SZ Cha can be found in Chamaeleon I, all of which are about 600 light-years from Earth. The discs have been scaled to appear roughly the same size in this composition. Credit: ESO\/C. Ginski, A. Garufi, P.-G. Valeg\u00e5rd et al.<\/p>\n The imagery is stunning. Per-Gunnar Valeg\u00e5rd (University of Amsterdam), who led the work on Orion, noted \u201cIt is almost poetic that the processes that mark the start of the journey towards forming planets and ultimately life in our own Solar System should be so beautiful.\u201d I think we can drop that word \u2018almost\u2019 as we are reminded that the sheer wonder of celestial vistas at ever increasing scales is what drew many of us into astronomy at an early age. We\u2019re a long way from those grainy Palomar images of Jupiter and Saturn that I acquired decades ago at Hayden Planetarium in Chicago.<\/p>\n Image:<\/strong> Planet-forming discs around young stars and their location within the gas-rich cloud of Chamaeleon I, roughly 600 light-years from Earth. The team observed 20 stars in the Chamaeleon I region, detecting discs around 13. The background image shows an infrared view of Chamaeleon I captured by the Herschel Space Observatory. Credit: ESO\/C. Ginski et al.; ESA\/Herschel.<\/p>\n Meanwhile, in a separate investigation of the young star T Chamaeleontis, we gain insights into planet formation in its latter stages. Here we have the James Webb Space Telescope to thank. Naman Bajaj (University of Arizona) and Uma Gorti (SETI Institute) have been able to extract data on the four lines of the noble gases neon (Ne) and argon (Ar), with the neon detection showing processes that are occurring over an extended area. The observations show dispersing gasses from a planet-forming disk that is in its latter stages, completing the formation process. These \u2018winds\u2019 seem to be driven by stellar photons or, according to Bajaj, by the magnetic field within the disk itself. <\/p>\n The find is significant, says Richard Alexander (University of Leicester):<\/p>\n \u201cWe first used neon to study planet-forming discs more than a decade ago, testing our computational simulations against data from Spitzer, and new observations we obtained with the ESO VLT. We learned a lot, but those observations didn\u2019t allow us to measure how much mass the discs were losing. The new JWST data are spectacular, and being able to resolve disc winds in images is something I never thought would be possible. With more observations like this still to come, JWST will enable us to understand young planetary systems as never before.\u201d<\/p>\n<\/blockquote>\n The paper on the history of transit studies is Briot & Schneider, \u201cPrehistory of Transit Searches,\u201d in Handbook of Exoplanets<\/em><\/a>, 2nd edition. <\/p>\n The papers on the SPHERE work are Ginski et al., \u201cThe SPHERE view of the Chamaeleon I star-forming region: The full census of planet-forming disks with GTO and DESTINYS programs\u201d, Astronomy & Astrophysics<\/em> (abstract<\/a>). <\/p>\n And two other papers in the same issue: Garufi et al., \u201cThe SPHERE view of the Taurus star-forming region: The full census of planet-forming disks with GTO and DESTINYS programs,\u201d (abstract<\/a>) and Valegard et al., \u201cDisk Evolution Study Through Imaging of Nearby Young Stars (DESTINYS): The SPHERE view of the Orion star-forming region\u201d (abstract<\/a>). <\/p>\n The paper on T Chamaeleontis is Bajaj et al., \u201cJWST MIRI MRS Observations of T Cha: Discovery of a Spatially Resolved Disk Wind,\u201d The Astronomical Journal<\/em> Vol. 167, No. 3 (4 March 2024), 127 (abstract<\/a>). <\/p>\n<\/p>\n\n
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