̽��ֱ�� of Cambridge - Institute for Advanced Study

Astronomers use ‘little hurricanes’ to weigh and date planets around young stars

sc604 — Fri, 06 Jan 2023 08:25:43 +0000

Researchers from the ̽��ֱ�� of Cambridge and the Institute for Advanced Study have developed a technique, which uses observations of these ‘hurricanes’ by the Atacama Large Millimeter/submillimetre Array (ALMA) to place some limits on the mass and age of planets in a young star system.

Pancake-like clouds of gases, dust and ice surrounding young stars – known as protoplanetary discs - are where the process of planet formation begins. Through a process known as core accretion, gravity causes particles in the disc to stick to each other, eventually forming larger solid bodies such as asteroids or planets. As young planets form, they start to carve gaps in the protoplanetary disc, like grooves on a vinyl record.

Even a relatively small planet – as small as one-tenth the mass of Jupiter according to some recent calculations – may be capable of creating such gaps. As these ‘super-Neptune’ planets can orbit their star at a distance greater than Pluto orbits the Sun, traditional methods of exoplanet detection cannot be used.

In addition to the grooves, observations from ALMA have shown other distinct structures in protoplanetary discs, such as banana- or peanut-shaped arcs and clumps. It had been thought that at least some of these structures were also driven by planets.

“Something must be causing these structures to form,” said lead author Professor Roman Rafikov from Cambridge’s Department of Applied Mathematics and Theoretical Physics, and the Institute for Advanced Study in Princeton, New Jersey. “One of the possible mechanisms for producing these structures – and certainly the most intriguing one – is that dust particles that we see as arcs and clumps are concentrated in the centres of fluid vortices: essentially little hurricanes that can be triggered by a particular instability at the edges of the gaps carved in protoplanetary discs by planets.”

Working with his PhD student Nicolas Cimerman, Rafikov used this interpretation to develop a method to constrain a planet’s mass or age if a vortex is observed in a protoplanetary disc. Their results have been accepted for publication in two separate papers in the Monthly Notices of the Royal Astronomical Society.

“It’s extremely difficult to study smaller planets that are far away from their star by directly imaging them: it would be like trying to spot a firefly in front of a lighthouse,” said Rafikov. “We need other, different methods to learn about these planets.”

To develop their method, the two researchers first theoretically calculated the length of time it would take for a vortex to be produced in the disc by a planet. They then used these calculations to constrain the properties of planets in discs with vortices, basically setting lower limits on the planet’s mass or age. They call these techniques ‘vortex weighing’ and ‘vortex dating’ of planets.

When a growing planet becomes massive enough, it starts pushing material from the disc away, creating the tell-tale gap in the disc. When this happens, material on the outside of the gap becomes denser than material on the inside of the gap. As the gap gets deeper and the differences in density become large, an instability can be triggered. This instability perturbs the disc and can eventually produce a vortex.

“Over time, multiple vortices can merge together, evolving into one big structure that looks like the arcs we’ve observed with ALMA,” said Cimerman. Since the vortices need time to form, the researchers say their method is like a clock that can help determine the mass and age of the planet.

“More massive planets produce vortices earlier in their development due to their stronger gravity, so we can use the vortices to place some constraints on the mass of the planet, even if we can’t see the planet directly,” said Rafikov.

Using various data points such as spectra, luminosity and motion, astronomers can determine the approximate age of a star. With this information, the Cambridge researchers calculated the lowest possible mass of a planet that could have been in orbit around the star since the protoplanetary disc formed and was able to produce a vortex that could be seen by ALMA. This helped them put a lower limit on the mass of the planet without observing it directly.

By applying this technique to several known protoplanetary discs with prominent arcs, suggestive of vortices, the researchers found that the putative planets creating these vortices must have masses of at least several tens of Earth masses, in the super-Neptune range.

“In my daily work, I often focus on the technical aspects of performing the simulations,” said Cimerman. “It’s exciting when things come together and we can use our theoretical findings to learn something about real systems.”

“Our constraints can be combined with the limits provided by other methods to improve our understanding of planetary characteristics and planet formation pathways in these systems,” said Rafikov. “By studying planet formation in other star systems, we may learn more about how our own Solar System evolved.”

̽��ֱ��research was supported in part by the Science and Technology Facilities Council (STFC), part of UK Research and Innovation (UKRI).

References:
Roman R Rafikov and Nicolas P Cimerman. ‘Vortex weighing and dating of planets in protoplanetary discs.’ Monthly Notices of the Royal Astronomical Society (2022). DOI: 10.1093/mnras/stac3692 or DOI: 10.48550/arXiv.2301.01789

Nicolas P Cimerman and Roman R Rafikov. ‘Emergence of vortices at the edges of planet-driven gaps in protoplanetary discs.’ Monthly Notices of the Royal Astronomical Society (2022). DOI: 10.1093/mnras/stac3507

Little ‘hurricanes’ that form in the discs of gas and dust around young stars can be used to study certain aspects of planet formation, even for smaller planets which orbit their star at large distances and are out of reach for most telescopes.

It’s extremely difficult to study smaller planets that are far away from their star by directly imaging them: it would be like trying to spot a firefly in front of a lighthouse. We need other, different methods to learn about these planets

Roman Rafikov

ALMA (ESO/NAOJ/NRAO)

ALMA image of the protoplanetary disc around HL Tauri

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Astronomers show how planets form in binary systems without getting crushed

sc604 — Mon, 26 Jul 2021 23:06:00 +0000

̽��ֱ��researchers, from the ̽��ֱ�� of Cambridge and the Max Planck Institute for Extra-terrestrial Physics, have shown how exoplanets in binary star systems – such as the ‘Tatooine’ planets spotted by NASA’s Kepler Space Telescope – came into being without being destroyed in their chaotic birth environment.

They studied a type of binary system where the smaller companion star orbits the larger parent star approximately once every 100 years – our nearest neighbour, Alpha Centauri, is an example of such a system.

“A system like this would be the equivalent of a second Sun where Uranus is, which would have made our own solar system look very different,” said co-author Dr Roman Rafikov from Cambridge’s Department of Applied Mathematics and Theoretical Physics, who is also a member at the Institute for Advanced Study in Princeton, New Jersey.

Rafikov and his co-author Dr Kedron Silsbee from the Max Planck Institute for Extra-terrestrial Physics found that for planets to form in these systems, the planetesimals – planetary building blocks which orbit around a young star – need to start off at least 10 kilometres in diameter, and the disc of dust and ice and gas surrounding the star within which the planets form needs to be relatively circular.

̽��ֱ��research, which is published in Astronomy and Astrophysics, brings the study of planet formation in binaries to a new level of realism and explains how such planets, a number of which have been detected, could have formed.

Planet formation is believed to begin in a protoplanetary disc – made primarily of hydrogen, helium, and tiny particles of ices and dust – orbiting a young star. According to the current leading theory on how planets form, known as core accretion, the dust particles stick to each other, eventually forming larger and larger solid bodies. If the process stops early, the result can be a rocky Earth-like planet. If the planet grows bigger than Earth, then its gravity is sufficient to trap a large quantity of gas from the disc, leading to the formation of a gas giant like Jupiter.

“This theory makes sense for planetary systems formed around a single star, but planet formation in binary systems is more complicated, because the companion star acts like a giant eggbeater, dynamically exciting the protoplanetary disc,” said Rafikov.

“In a system with a single star the particles in the disc are moving at low velocities, so they easily stick together when they collide, allowing them to grow,” said Silsbee. “But because of the gravitational ‘eggbeater’ effect of the companion star in a binary system, the solid particles there collide with each other at much higher velocity. So, when they collide, they destroy each other.”

Many exoplanets have been spotted in binary systems, so the question is how they got there. Some astronomers have even suggested that perhaps these planets were floating in interstellar space and got sucked in by the gravity of a binary, for instance.

Rafikov and Silsbee carried out a series of simulations to help solve this mystery. They developed a detailed mathematical model of planetary growth in a binary that uses realistic physical inputs and accounts for processes that are often overlooked, such as the gravitational effect of the gas disc on the motion of planetesimals within it.

“ ̽��ֱ��disc is known to directly affect planetesimals through gas drag, acting like a kind of wind,” said Silsbee. “A few years ago, we realised that in addition to the gas drag, the gravity of the disc itself dramatically alters dynamics of the planetesimals, in some cases allowing planets to form even despite the gravitational perturbations due to the stellar companion.”

“ ̽��ֱ��model we’ve built pulls together this work, as well as other previous work, to test the planet formation theories,” said Rafikov.

Their model found that planets can form in binary systems such as Alpha Centauri, provided that the planetesimals start out at least 10 kilometres across in size, and that the protoplanetary disc itself is close to circular, without major irregularities. When these conditions are met, the planetesimals in certain parts of the disc end up moving slowly enough relative to each other that they stick together instead of destroying each other.

These findings lend support to a particular mechanism of planetesimal formation, called the streaming instability, being an integral part of the planet formation process. This instability is a collective effect, involving many solid particles in the presence of gas, that is capable of concentrating pebble-to-boulder sized dust grains to produce a few large planetesimals, which would survive most collisions.

̽��ֱ��results of this work provide important insights for theories of planet formation around both binary and single stars, as well as for the hydrodynamic simulations of protoplanetary discs in binaries. In future, the model could also be used to explain the origin of the ‘Tatooine’ planets – exoplanets orbiting both components of a binary – about a dozen of which have been identified by NASA’s Kepler Space Telescope.

Reference:
Kedron Silsbee and Roman R. Rafikov. ‘Planet Formation in Stellar Binaries: Global Simulations of Planetesimal Growth.’ Astronomy and Astrophysics (2021). DOI:10.1051/0004-6361/20214113

Astronomers have developed the most realistic model to date of planet formation in binary star systems.

Planet formation in binary systems is more complicated, because the companion star acts like a giant eggbeater, dynamically exciting the protoplanetary disc

Roman Rafikov

ESO/L. Calçada/N. Risinger

Artist’s impression of the planet around Alpha Centauri B

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