Disorderly planetary system challenges the theories

The planets orbiting our own Sun do so in an orderly system. Rocky planets are on the inside, while the gas giants – including Jupiter – are on the outside, and all the trajectories follow virtually the same plane. This is how it most often commonly takes place, according to the astronomers’ theories for planet formation. However, a newly discovered planetary system is challenging the theories.

2013.10.17 | Rasmus Rørbæk

When a newly born star shines for the first time, it is surrounded by a disc of large amounts of gas and dust. In simple terms, these are the remains the star itself is unable to contain. Over a period of many years, these remains become slowly drawn together as planets that organise themselves in orbit around the star, with rocky planets on the inside and gas giants on the outside. In any case – until now – this was the simple concept of the formation of planetary systems. The discovery of a new planetary system – Kepler-56 – actually now compels astrophysicists to critically evaluate the theory. (Illustration: NASA/JPL-Caltech/T. Pyle, (NASA Spitzer Science Center)

This illustration shows the planets’ trajectories in relation to the Kepler-56 star’s rotation axis. In a ‘normal’ system, the star’s rotation (seen here as an arrow through the star) points in a more upward and downward direction, and the planets’ trajectories are on the same plane as the star’s equator. The star is a 3.5 billion-year-old red giant in the final stage of its development. It is located about 1000 light years from Earth and has a diameter that is four times the size of the Sun’s. The two ‘small’ planets are called Kepler-56b and 56c, and they are both large planets: 56b is 22 times the mass of the Earth and 56c is 180 times the mass of the Earth. The newly discovered planet – 56d – is the size of Jupiter and is visible at the right. Its trajectory and the angle of the star’s rotation axis in relation to the planetary orbits are puzzling astrophysicists because they do not keep to the ‘rules’ that were previously thought to apply to the formation of solar systems. (Illustration: Daniel Huber, NASA/Ames)

An international research group with the participation of Aarhus scientists has observed for the first time a planetary system in which the host star has a misaligned rotation axis with respect to the orbit of the planets. The discovery was recently published in the scientific journal Science with some intriguing new thoughts about one of the oldest mysteries in exoplanetary research: the formation of hot Jupiters – enormous gas planets that orbit extremely close to their host stars.

The discovery has more or less shaken up the foundation of the astrophysicists’ theory regarding how solar systems are formed, because this system is very different from our own – and virtually all other known systems for that matter.

“Nearly two decades after the discovery of the first hot Jupiter, we’ve now found an important element in the understanding of how planetary systems are created. What makes this discovery so fantastic is that the system has a very strange imbalance between the star and the trajectory of the planets, and we can’t explain it yet. We can nevertheless say without hesitation that this will make us question a number of theories,” explains Professor Jørgen Christensen-Dalsgaard, Director of the Stellar Astrophysics Centre at Aarhus University.

Why is this so weird?

In the four years during which the Kepler space telescope has been aimed at a field of stars, the astrophysicists have had their breath taken away on several occasions. We now know that there are water planets, ultra-dark planets, systems with several suns and roughly speaking all the other amazing visions we are familiar with from science fiction films – and not least, that there is a vast myriad of different planets out there.

But this system – known as Kepler-56 – is the first that has seriously questioned the simple notions we have had about how planetary systems are formed.

Until now, the theory has been that planets are formed from the remains a star leaves in a cloud surrounding itself when it is first ‘born’. In this way, the cloud of gas and dust is a kind of ‘planet incubator’ in which all the material that turns into planets clusters together over a period of time and arranges itself in orbits roughly around the star’s equator, with rocky planets on the inside and gas giants on the outside – much like that we know from our own system.

“It’s not in the least like this with Kepler-56, and that’s what makes it quite exceptional. In the Kepler-56 system, we actually see a planet the size of Jupiter with a quite different trajectory compared with the two innermost. This shouldn’t be possible according to the common theory,” explains Associate Professor Hans Kjeldsen, Department of Physics and Astronomy, Aarhus University.

By carrying out very precise measurements of the star’s surface vibrations, the researchers have discovered that Kepler-56 has an approximately 47° angle – or misalignment in other words – in its rotation axis compared with the orbit of the innermost planets.

“It’s these misalignments between the planetary orbits and the star’s angle of rotation that are now making us take a critical look at the simple model of the formation of the systems. Everything indicates that this system has had an extremely chaotic existence. The newly discovered system is a very disorderly one,” says Associate Professor Kjeldsen.

Kepler space probe’s legacy

The discovery of the new planetary system is partly the result of the extensive expertise gathered in the international Kepler project, and partly due to the unprecedented amount of data collected by the Kepler satellite in the four years it has been on its mission. The probe can certainly no longer gather information the way it did, but the library it has delivered to the astrophysicists still contains invaluable data waiting to be read. The data could very well contain challenges to more theories and answers to more questions.

“This discovery is a beautiful example of the Kepler mission continuing to have considerable value and relevance for research into the universe. The library of data regularly supplies so much new knowledge that the prerequisites for understanding the existing theory are changing. As we gradually get better at analysing the information we’ve received from the telescope, it’s my hope that we’ll find even more surprises out there,” says Professor Christensen-Dalsgaard.

In addition to Jørgen Christensen-Dalsgaard and Hans Kjeldsen, their colleagues Torben Arentoft, Christoffer Karoff, Mikkel Nørup Lund, Travis Scott Metcalfe, Victor Silva Aguirre and Mia Lundquist, all from Aarhus University, and Rasmus Handberg, now Birmingham, are co-authors of the Science article entitled Stellar Spin-Orbit Misalignment in a Multiplanet System.

For more information, please contact

Associate Professor Hans Kjeldsen
Department of Physics and Astronomy
Aarhus University
+45 2338 2160
hans@phys.au.dk

Professor Jørgen Christensen-Dalsgaard
Department of Physics and Astronomy
Aarhus University
+45 2338 2374
jcd@phys.au.dk

Read more (in Danish only) about the importance of Kepler to date for astrophysics and human understanding of the universe here.

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