What do we know about the birth of planets?
Astrophysicists have been working for decades on one of the questions that still captures the imagination - how are planets formed? Although theories have existed for many years, it is only now as new technology and more powerful telescopes emerge that researchers are beginning to get to the bottom of the process.
One such researcher is Dr Farzana Meru, an astrophysicist at the University of Warwick who is an expert on the study of protoplanetary discs – the birth sites of planets. She explains what we know - and what we don’t...yet.
First – you need a star
A star is born out of a cloud of dust and gas, called a molecular cloud. This cloud is rotating and collapsing at the same time and the end result is that a disc forms around the star, which we call a protoplanetary disc. We are beginning to understand these discs more and more and it is within these discs that planets are born.
Protoplanetary discs were first seen from telescopes a few decades ago. Back then it was just a blobby halo in a pixelated image. But now we have entered a revolutionary era in protoplanetary disk studies.
Rings and gaps
With technology improving all the time, we are getting much more detailed images of the beautiful formations of protoplanetary discs. They come in a range of patterns – from spirals to concentric circles. By far the most common type of pattern is a series of rings and gaps in the disc. Astronomers think these patterns may indicate different stages in planet development or the different make up of gases and dust materials around the star.
We know planets form within protoplanetary discs, but there are two theories as to the process.
The main way we think formation occurs is through ‘Core Accretion’. If you think about the dust in the disk floating about and colliding with other particles, it may stick together and over time it grows to form a planetary core. As the core gathers more mass, its gravity pulls yet more dust and gas towards it, and eventually forms a terrestrial planet. If the core can gain enough mass it starts rapidly gathering gas, forming a gas giant planet.
Another way in which planets could form is through the second theory - Gravitational Instability. This idea is based on the theory that in the early stages of a protoplanetary disc’s life, it is so massive that its own gravity causes spirals to form — like the spiral features in a galaxy. If the gravity in the spirals is strong enough, they can become unstable and break apart into gas balls. Eventually dust collects in the middle of these gas balls and they grow to form giant planets.
Combining observations and theory
With more and improving images of protoplanetary discs we can begin to model how planets may form. For example, we may be able to tell from the size of the gaps, what mass of planet may be forming. My research uses computer simulations to understand how planets form and evolve, and makes the connection with observations. Recently I have focussed on understanding the processes that operate in protoplanetary discs to see if there are clear signatures that observers can look out for when looking at these discs.
Although it is now an accepted theory, there have only been a couple of instances when planets have been detected within the discs.
Firstly, when they have been indirectly observed, where the planet can’t be seen, but know it is there because its gravitational interaction with the star causes the star to move. While doing so, the colour of the light that the star gives off changes over time. When we see that happening periodically, we can conclude that there is a planet there.
But then last year an image of what appears to be a planet forming in the disc was captured by the SPHERE instrument on the Very Large Telescope in Chile for the first time.
It is hugely exciting to know that more images are sure to follow.
The number of planets is growing
Scientists are discovering new exoplanets all the time and we are getting a more diverse range than those that exist within our own solar system. For example in the last few years we have found Super-Earths – terrestrial planets that are approximately ten times more massive than our own Earth. There are also Hot Jupiters – Jupiter sized gas planets which are close in to their central star.
The ultimate goal would be to see from the patterns in the protoplanetary disc just what type of planet is evolving and what stage it is at in its development. We know that the disc lasts for only a few million years after the star and disc form, but fully formed planets are a few billion years old. That is a big period of time to fill but making the connection between the early stages of planet formation and the observed planets is an interesting challenge that astronomers are working on!
17 October 2019
Dr Farzana Meru is Assistant Professor and Royal Society Dorothy Hodgkin Fellow in the Astronomy & Astrophysics group at Warwick. Her research is focussed on planet formation, planet evolution and disc evolution, mostly using a variety of numerical techniques. She runs hydrodynamical simulations, using high performance computing facilities, to understand how planets form and evolve in young protoplanetary discs.
Institute of Astronomy — Amanda Smith & Farzana Meru
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