https://www.centauri-dreams.org/2025/04/04/protoplanetary-disks-are-smaller-than-expected/ Centauri Dreams * Home * About * Administrative * Booklist * Contact Select Page [ ] Protoplanetary Disks Are Smaller Than Expected by Paul Gilster | Apr 4, 2025 | Exoplanetary Science | 3 comments In astronomy, the first thing you see may be the least typical. A case in point: 'Hot Jupiters.' A few prescient souls, among them Buzz Aldrin and John Barnes in their novel Encounter with Tiber, speculated about gas giants that survived incredibly tight orbits around their star, and when asked about this in the 1990s, Greg Matloff ran the numbers and confirmed to his surprise that there was a theoretical case for their existence. Let me quote Matloff on this: Although I was initially very skeptical since then-standard models of solar system formation seemed to rule out such a possibility, I searched through the literature and located the appropriate equation (Jastrow and Rasool, 1965)....To my amazement, Buzz was correct. The planet's atmosphere is stable for billions of years. Since I was at the time working as a consultant and adjunct professor, I did not challenge the existing physical paradigm by submitting my results to a mainstream journal. Since "Hot Jupiters" were discovered shortly before the novel was published, I am now credited with predicting the existence of such worlds. 51 Pegasi b made the case for 'hot Jupiters' convincingly in 1995, and we also know that the reason we began finding these planets was that our best methods for detecting exoplanets in the mid-1990s involved radial velocity and transit measurements, both of which are most effective when dealing with large planets in close orbits. As best I can determine, current estimates find a scant ~0.5-1% of these planets around F-, G-, or K-class stars. The rarity of hot Jupiters like TOI-4201b underlines how rare these objects are around M-dwarfs. Observational bias can be a tricky thing. So is something of the same phenomenon occurring in our study of how planets form? Protoplanetary disks are extensively studied, some of them showing what appear to be planets in their early stages of formation. As I've read studies about disks like those at TW Hydrae and HL Tauri, I've always assumed that they were massive. In some cases they extend far beyond our Solar System's outer reaches. But now we have a new study making a strong case that disks like these are not typical. The work in question involved data from ALMA, the Atacama Large Millimeter/submillimeter Array. It comprises data from observations from 2023 and 2024 at resolutions of 0.030 arcsecond, along with somewhat higher resolution archival data to produce a high resolution survey of an entire star-forming region. The imaging takes in all known protoplanetary disks around young stars in the Lupus star forming region, some 400 light years from Earth. The results indicate how much we have to learn about protoplanetary disks. The smallest disk comes to only 0.6 astronomical units in radius, which means it is smaller than the Earth's orbit. The researchers also learned that the kind of features that point to planets in formation do not occur in smaller disks. Thus lead author Osmar M. Guerra-Alvarado (Leiden University, The Netherlands): "These results completely change our view of what a 'typical' protoplanetary disc looks like. Only the brightest discs which are the easiest to observe show large-scale gaps, whereas compact discs without such substructures are actually much more common." [discs-ordered-credit-Guerra-Alvarado-et-al] Image: Images of 73 protoplanetary discs in the Lupus star forming region (two of the images contain binary stars). Only a fraction of the discs extend beyond the orbit of Neptune, when compared to our own Solar System. Most of the observed discs are small and show no structures like gaps and rings. (c) Guerra-Alvarado et al. Adds Leiden colleague Nienke van der Marel: "The discovery that the majority of the small discs do not show gaps, implies that the majority of stars do not host giant planets. This is consistent with what we see in exoplanet populations around full-grown stars. These observations link the disc population directly to the exoplanet population." The smaller protoplanetary disks are found around stars massing between 10 and 50 percent of the mass of the Sun. While disks like these do not appear optimum for the formation of gas giants, they may be fertile breeding ground for super-Earths, with most of the dust located close to the host star. Given this context, our own Solar System may have been formed from a more massive protoplanetary disk large enough to produce a Jupiter and a Saturn. [eso1436f] Image: This is an artist's impression of a young star surrounded by a protoplanetary disc in which planets are forming. Using ALMA's 15-kilometre baseline astronomers were able to make the first detailed image of a protoplanetary disc, which revealed the complex structure of the disc. Concentric rings of gas, with gaps indicating planet formation, are visible in this artist's impression and were predicted by computer simulations. Now these structures have been observed by ALMA for the first time. Note that the planets are not shown to scale. Credit: ESO/L. Calcada. From the paper: The fact that we observe numerous compact disks in the Lupus region, both with and without substructures, aligns with the scenario from van der Marel & Mulders (2021), in which such disks, undergoing significant radial drift, can supply enough dust material to form multiple super-Earths (Sanchez et al. 2024). This offers a potential explanation for the origin of the exoplanet populations observed around M-stars to this day. In addition, the fact that we are still observing these disks may require something to halt dust drift and trapping particles. Instead of forming a single Jupiter or Saturn-mass planet, several super-Earths or smaller planets may be forming in the inner regions of these disks, collectively stopping the drift (Huang et al. 2024b). So we have to look at the question of observation bias once again. As with hot Jupiters, our best methods continue to improve, and as they do, we gradually form a broader picture of the phenomena being studied. In the case of the latest ALMA work, we are delving into higher-resolution measurements that can begin to tease out information about small disks that have previously been studied only in terms of brightness as opposed to size. A 'typical' disk turns out to be something other than the large, bright disks first revealed by ALMA. The paper is Guerra-Alvarado et al., "A high-resolution survey of protoplanetary discs in Lupus and the nature of compact discs," Accepted for publication in Astronomy & Astrophyiscs (preprint). [tzf_img_post] 3 Comments 1. Michael Fidler on April 4, 2025 at 23:07 How Many Rogue Planets Are There in the Milky Way? New simulations suggest that many, if not most, planets are ejected from their stars early in their formation. https://www.scientificamerican.com/article/ how-many-rogue-planets-are-in-the-milky-way/ https://arxiv.org/pdf/2501.13166 This raises the question: How many of these rogue planets might be habitable? Super-Earths, for example, could generate enough internal heat and possess a sufficient atmosphere to insulate these wandering planets, allowing for the possibility of life to evolve. Large eyes would be a real asset on these nocturnal worlds... Reply 2. Michael on April 5, 2025 at 4:17 I am not surprised, I think once the inner planets have formed the interaction between them, outflowing gas and dust throws them out further so the system looks bigger than they are. Reply 3. KB on April 5, 2025 at 8:00 It's a bit unfortunate the paper does not explicitly give statistics versus the host star mass (I've only skimmed it so apologies if I've missed this). I guess we have to assume that disk size scales with host star mass? Reply Submit a Comment Cancel reply Your email address will not be published. Required fields are marked * [ ] [ ] [ ] [ ] [ ] [ ] [ ] Comment * [ ] Name * [ ] Email * [ ] Website [ ] [ ] Save my name, email, and website in this browser for the next time I comment. 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Now Reading [kepler5] Recent Posts * Protoplanetary Disks Are Smaller Than Expected * A Multiwavelength Look at Proxima Centauri's Flares * Can an Interstellar Generation Ship Maintain a Population on a 250-year Trip to a Habitable Exoplanet? * The Ethics of Spreading Life in the Cosmos * Why Do Super-Earths and Mini-Neptunes Form Where They Do? * Deep Space and the Art of the Map * Are Supernovae Implicated in Mass Extinctions? * Quantifying the Centauri Stream On Comments If you'd like to submit a comment for possible publication on Centauri Dreams, I will be glad to consider it. The primary criterion is that comments contribute meaningfully to the debate. Among other criteria for selection: Comments must be on topic, directly related to the post in question, must use appropriate language, and must not be abusive to others. Civility counts. In addition, a valid email address is required for a comment to be considered. 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