≡ Menu

Simultaneous Growth of Planet & Star?

I’m interested in a new paper on planet formation, not only for its conclusions but its methodology. What Amy Bonsor (University of Cambridge) and colleagues are drawing from their data is how quickly planets can form. We’ve looked numerous times in these pages at core accretion models that explain the emergence of rocky worlds and gravitational instability models that may offer a way of producing a gas giant. But how long after the formation of the circumstellar disk do these classes of planets actually appear?

A planet like the Earth poses fewer challenges than a Jupiter or Saturn. Small particles run into each other within the gas and dust disk surrounding the young star, assembling planets and other debris through a process of clumping that eventually forms planetesimals that themselves interact and collide. Thus core accretion: The planet ‘grows’ in ways that are readily modeled and can be observed in disks around other stars.

But the gas giants still pose problems. Core accretion would suggest the growth of a solid core that gradually draws in mass until a dense atmosphere enshrouds it. But the core accretion process, according to the latest models, takes long enough that by the time it has finished, the disk is depleted. Gas giants are primarily made of hydrogen and helium, but these gases disappear from the disk in relatively short order. So a gas giant’s formation has to occur quickly and early, before the needed hydrogen and helium are blown off by radiation from the young star or consumed by it.

Gas giants around M-dwarfs may be one route to follow here, for core accretion seems to operate more slowly in M-dwarf systems, and we almost have to call in relatively short-order clumping – this is the disk instability model championed by Alan Boss – to explain how such worlds could form. So one observational path into gas giant formation is to look for such worlds around smaller stars, where their presence would indicate a model other than core accretion at work. But Bonsor and team have chosen an ingenious alternate route: They’re probing the atmospheres of white dwarf stars.

Bonsor points out that “[s]ome white dwarfs are amazing laboratories, because their thin atmospheres are almost like celestial graveyards.” Exactly so, because so-called ‘polluted’ white dwarfs show clear signs of heavy elements like magnesium, iron and calcium in their atmospheres, and the assumption is that these elements must be the result of small bodies within the stellar system falling into the parent star. So the method here is to use spectroscopic observations to probe the composition of asteroids that are long gone, but whose traces help us chart the conditions of their formation.

Studying the atmospheres of more than 200 polluted white dwarfs, the researchers found that the elements there can only be explained by the infall of asteroids that have undergone differentiation. In other words, they have gone through the process of melting, with iron sinking into their core while lighter elements rise to the surface. Amy Bonsor explains:

“The cause of the melting can only be attributed to very short-lived radioactive elements, which existed in the earliest stages of the planetary system but decay away in just a million years. In other words, if these asteroids were melted by something which only exists for a very brief time at the dawn of the planetary system, then the process of planet formation must kick off very quickly.”

Image: This is Figure 3 from the paper. Caption: The core- or mantle-rich materials in the atmospheres of white dwarfs are the collision fragments of planetesimals that formed earlier than ∼1 Myr, when large-scale melting was fueled by the decay of 26Al. Alternatively, in the most massive, close-in, highly excited, planetesimal belts, catastrophic collisions between Pluto-sized bodies (anything with D > 1, 400 km) could supply most smaller planetesimals. Gravitational potential energy during accretion can fuel large-scale melting and core formation in these large bodies, such that almost all planetary bodies in the belt are the collision fragments of core–mantle differentiated bodies. tMS , tGB and tWD refer to the star’s main-sequence, giant branch lifetimes and the start of the white dwarf phase. Credit: Bonsor et al.

The researchers’ simulations of planetesimal and collisional evolution show that short-lived radioactive nuclides like Aluminium-26 (26Al) are the most likely heat source to explain the accreted iron core or mantle material. From the paper:

The need for enhanced abundances of 26Al to explain core- or mantle-rich white dwarf spectra provides distinct evidence for the early formation of planetesimals in exoplanetary systems contemporaneously with star formation. Rapid planetesimal formation offers an explanation for the difference in mass budgets between Class 0, I and II discs [6]. Our findings point to the growth of large, > 10 km-sized planetesimals, potentially even planetary cores, rather than just the coagulation of pebbles. The earlier planetary cores form, the more likely they are to grow to the pebble isolation mass and the more likely giant planet formation is to occur early-on, which can provide an explanation for substructures commonly observed with ALMA.

Early planet formation helps explain how gas giant planets form and seems to put pressure on gravitational instability models, although there may be multiple routes to the same result. But it is striking that the researchers, who include scientists at Oxford, the Ludwig-Maximilians-Universität in Munich, the University of Groningen and the Max Planck Institute for Solar System Research, Gottingen, find evidence for the early formation of planetesimals “contemporaneously with star formation.”

Thus star and planet formation begin concurrently, under this model, with planets evolving during the collapse of the circumstellar disk. I’ve always found white dwarfs fascinating, but that we might probe the origins of stellar systems by analyzing the composition of their atmospheres is remarkable. It points to the continuing vitality of work on this class of star for understanding both planetary and stellar evolution.

The paper is Bonsor et al. “Rapid formation of exoplanetesimals revealed by white dwarfs,”’ Nature Astronomy 14 November 2022. Abstract.

{ 6 comments… add one }
  • Geoffrey Hillend November 15, 2022, 17:10

    Very informative paper. Recently, I thought that maybe gas giants might form early in the accretion disk. I like the idea that the gas giants had to form early otherwise the stars wind blows away the hydrogen and helium. Also it makes sense that the planetesimals formed early and there cores differentiated and we see that from the heavy elements in the spectra of white dwarfs which is the result of small bodies which are destroyed or vaporized by their collision with the white dwarf caused by the kinetic energy from the white dwarfs gravitational field. This is a good idea since it makes sense that all bodies in an accretion disk form early is time based, a testable hypothesis and matches observations.

    • Jeff Wright November 20, 2022, 6:57

      System-building…a community effort.

  • James Franklin November 15, 2022, 17:44

    I still believe they are vastly unplaying the role electrostatics plays in the formation of any body, including stellar mass objects. Ionisation of molecules and particles will occur as a result of interstellar radiation – what we call cosmic rays – and those molecules and particles will be a mixture of positively and negatively charged – we see this under our settee’s, behind cupboards, fridges etc, as positively and negatively charged dust materials particles form clumps – and this is in a static and stable environment – the environment in an interstellar dust cloud that has been impacted by an outside force, such as a passing star or a supernova shock front, will be chaotic at best – a perfect environment for positively and negatively charged particles to find each other, en- mass, and begin the process of core building with fervour.

    This process is likely also how stars start to form, with the whole cloud rotating and the part of the cloud that forms a core first likely keeps pulling in material until it flashes into life as a star – the speed of the inflow of material likely helps to determine the mass of the star – the faster material flows inward, the pressure of the core is required to be larger to counter this inflow – I am sure other factors are major players too.

    As this material is inflowing to the centre, material in orbit of this core will form unstable parts were the same process starts – simultaneously with the main core formation – and this could help to explain the gas giants and even brown dwarf stars we see around stars of all mass ranges – the faster the material flows into that core, the larger the planet can become, especially once it reaches a mass were gravity becomes a significant and decisive element.

    • Geoffrey Hillend November 16, 2022, 18:38

      Actually, the dust gets attracted to the dampness of the refrigerator coils and cupboards since dust is hydrophilic or water loving. The electromagnetic forces works only short distance which is a problem or dust clouds in the ISM. Like charges repel each other and so do dust particles in the ISM. The gravity has to over power the electrostatic forces between the dust before it can collapse the dust cloud. Fortunately, gravity works over long distances. Magnetic fields do remove some of the angular momentum from the proto stellar cloud during star formation, the collapse of the cloud. I don’t remember the reference source, so I can’t site a reference.

  • Michael C Fidler November 15, 2022, 21:01

    The formation of these stars and planets and the amount of Aluminium-26 that is available depends on the gas/plasma/planetoid that they form in. Supernova production is the main contributor to 26Al so that would depend on how long before the star system forms after the the supernova cloud is produced. Are areas going to be more productive in 26Al? This could be like agriculture where certain fertile soils produce large crops or are conducive to certain plants. Age and conditions in relation to supernovas are the factors here, like volcanic soils and fertility.

  • Michael C Fidler November 15, 2022, 21:24

    Interesting NASA group called Clever Planets relating to planetary accretion.

    Early planetary migration can explain missing planets.

    Earth isn’t ‘super’ because the sun had rings before planets.


Leave a Comment