For generations, planet formation was only a theory. As 2018 comes to an end, here’s the evidence of what’s going on.
The theory of planet formation has been around for a long time, but lacked validation.
Artist’s impression of a young star surrounded by a protoplanetary disk. There are many unknown properties about protoplanetary disks around Sun-like stars, but they all exhibit infrared radiation. Tabby’s star has none. (ESO/L. CALÇADA)
In principle, gas collapses to form protostars surrounded by protoplanetary disks.
The very young protostar M17-SO1, as imaged way back in 2005 with the ground-based Subaru telescope, shows features of a protoplanetary disk around a newly-forming star, but internal features were unable to be resolved with instrumentation of that time. (SUBARU / NAOJ)
As protostars grow, they heat up, while their disks race to form planets before the volatile material evaporates.
30 protoplanetary disks, or proplyds, as imaged by Hubble in the Orion Nebula. Hubble is a brilliant resource for identifying these disk signatures in the optical, but has little power to probe the internal features of these disks, even from its location in space. (NASA/ESA AND L. RICCI (ESO))
With observatories like Hubble, we’ve found and identified many disks, but couldn’t measure their internal properties.
This artist’s rendition, of a protoplanetary disk such as the one expected around TW Hydrae, shows that even with the best optical and near-infrared telescopes we have, we can only hope to infer the locations of the most prominent, massive planets forming in these protoplanetary environments. (NAOJ)
In theory, those disks ought to display gaps where massive, early planets have begun their formation.
A large number of protoplanetary systems have been imaged, but the state-of-the-art infrared imager designed for exoplanet disk pictures is SPHERE, which routinely obtains resolutions of ~10″, or less than 0.003 degrees per pixel. These protoplanetary disk images provide clues as to the planets forming inside them. (SHINE (SPHERE INFRARED SURVEY FOR EXOPLANETS) COLLABORATION / ARTHUR VIGAN)
At the Very Large Telescope, the SPHERE instrument successfully imaged a number of protoplanetary disks directly.
The observational structure of the young star MWC 758, at right, compared with a simulation involving a large outer planet, at left. This Herbig star is much more massive than our Sun ever was, but also is not a true star. (NASA, ESA, ESO, M. BENISTY ET AL. (UNIVERSITY OF GRENOBLE), R. DONG (LAWRENCE BERKELEY NATIONAL LABORATORY), AND Z. ZHU (PRINCETON UNIVERSITY))
Some displayed spirals due to massive outer planets, while others possessed symmetric rings caused by lower-mass worlds.
Eight young T Tauri stars, as imaged by SPHERE, show disks, rings, and symmetric, unperturbed structures. These 8 disks range in age from 1 to 15 million years, and are all around stars of 2 solar masses or less. (H. AVENHAUS ET AL. (2018), ARXIV.ORG/ABS/1803.10882)
The best portraits of protoplanetary disks, however, arise from ALMA.
Meteor, photographed over the Atacama Large Millimeter/sub-millimeter Array, 2014. ALMA is perhaps the most advanced and most complex array of radio telescopes in the world, is capable of imaging unprecedented details in protoplanetary disks, and is also an integral part of the Event Horizon Telescope. (ESO/C. MALIN)
ALMA’s crisp images are striking.
The distance from the young, central star determines the type of material that’s present. Heat and energy flux changes everything in these systems. The gaps in the rings and disk indicate the likely presence of planets, which are details that ALMA can reveal. (K. ZHANG IN G. A. BLAKE’S RESEARCH GROUP, FROM GEOFFREY A. BLAKE & EDWIN A. BERGIN, NATURE 520, 161–162 (09 APRIL 2015))
Its Disk Substructures at High Angular Resolution Project (DSHARP)
has just released their first results, revealing 20 nearby protoplanetary disks. These 20 protoplanetary disks, as they appear in the most recent ApJ letters paper (in press), showcase the diversity and intricate details found in both face-on and tilted protoplanetary disks imaged by the DSHARP team. (S. M. ANDREWS ET AL. AND THE DSHARP COLLABORATION, ARXIV:1812.04040)
Most have gaps, rings, and easily-identifiable locations where candidate planets may lie.
HD 163296 is representative of a typical protoplanetary disk viewed by the DSHARP collaboration. It has a central protoplanetary disk, outer emission rings, and gaps between them. There ought to be multiple planets in this system, and one can identify an odd artifact interior to the 2nd-from-the-outermost ring that may be a telltale sign of a perturbing planet. The scale bar at lower right is 10 AU, and appears in all DSHARP images shown here. (S. M. ANDREWS ET AL. AND THE DSHARP COLLABORATION, ARXIV:1812.04040)
We’ve already learned that the presence of such small-scale attributes are ubiquitous.
The protostar IM Lup has a protoplanetary disk around it that exhibits not only rings, but a spiral feature towards the center. This may or may not be a multi-star system, but there is likely a very massive planet causing these spiral features. That theoretical explanation, although compelling, has yet to be definitively confirmed. (S. M. ANDREWS ET AL. AND THE DSHARP COLLABORATION, ARXIV:1812.04040)
The most common features are the concentric emission rings and dust-depleted gaps.
The protostar HD 143006 displays two clearly concentric outer rings and a large interior gap. There may be a suite of planets located there as well as one in the gap between the outer rings. The ‘hot spot’ in the outermost ring may hint at the presence of an outer planet that tugs on the ring, or something else entirely. (S. M. ANDREWS ET AL. AND THE DSHARP COLLABORATION, ARXIV:1812.04040)
Understanding planetary evolution, from nebulae to protoplanets to full-blown solar systems, is finally within reach.
The protoplanetary disk around the star HL Tauri in a young star cluster may well be the best analogue of a Sun-like star forming, with planets around it, that we’ve ever seen. This was ALMA’s first protoplanetary disk to display the rings and gaps, and over the past 4 years our knowledge of protoplanetary evolution has taken us ever closer to a complete understanding of these systems. (ALMA (ESO/NAOJ/NRAO)/NASA/ESA) Mostly Mute Monday tells the astronomical story of objects, phenomena, or observational results in images, visuals, and no more than 200 words. Talk less; smile more. Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.