However, the discovery of planets around distant stars in the 1990s made it clear that the picture was much more complex than we knew. In new research (opens in new tab), we found a hot gas giant resembling Jupiter in the process of forming around a star about 500 light-years from Earth. This rare baby of a planet in the process of forming, drawing matter from a huge disk of dust and gas swirling around its also baby sun, has opened a window into mysteries that have puzzled astronomers for years.
Scientific triumph?
Scientific research into the origins of Earth and the other planets in our solar system began in the mid-17th century. Based on the work of the Swedish thinker Emanuel Swedenborg, the famous German philosopher Immanuel Kant suggested that the sun and its small planetary family evolved from a large rotating primordial cloud. Kant called this “Urnebel” in German for the nebula. This idea was later refined by the French scholar Pierre Laplace and has since had many more additions and revisions, but modern scientists believe it was basically on the right track. The modern descendant of Kant’s hypothesis, now full of detailed physics, can explain most of the observed features of our solar system. “Primary clouds” of dust and gas that form planets in the Orion Nebula. (Image credit: CR O’Dell / Rice University, NASA) We can now run computer simulations with all the correct settings and a beautiful digital copy of our solar system will appear. It will have the right kind of planets in the right orbits that clockwise, just like the real thing. This model is a triumphant composition of threads from geology, chemistry, physics and astronomy, and seemed to have covered bases. Until, that is, astronomers dealt with planets outside our solar system.
Beyond the solar system
When the first planetary systems orbiting distant stars were discovered in the mid-1990s, there was immediate controversy and concern. The new planets did not fit the model at all: the rest of the world, as it turned out, did not care so much about what happened here around our little sun. Since then, there has been an awareness that there may be different paths to forming a planetary system. Among the thousands of planets orbiting other stars now inhabiting our catalogs, our solar family of planets is beginning to look somewhat unusual. However, one of the most basic natural components of the planet-building mechanism is believed to be responsible for the formation of giant gas planets such as Jupiter and Saturn have stood the test of time: the idea of ”nucleus augmentation”. The accumulation of the nucleus begins with the gases and tiny dust grains believed to be Kant’s typical primordial cloud (which is shaped like a flattened rotating disk with the baby star in the center). The dust grains are collected in successively larger grains, then pebbles, stones and on a waterfall in baby planets or “planet-microns”. When such a cluster becomes large enough, it reaches a tipping point. Gravitational pull now helps the fetal planet to quickly draw gas, dust, and other clusters, clearing its orbital path and creating a circular vacuum in the disk. It is one of the hallmarks of modern astronomy that it is precisely the kinds of “disk space” predicted by theory that are now being seen and studied in the world.
Great pity
However, there are some things that the basic increment cannot explain. Huge planets have been located in orbit away from the stars that host them, in the cold distant point. According to the nucleus accretion theory, such planets should not exist. They are very far away, where the orbits move too slowly to operate the planet-building project. A new model of “gravitational collapse” was formulated to explain these unexpectedly huge distant planets (opens in a new tab). The basic idea is that if the primitive disk itself has enough mass, the whole thing can become unstable and collapse to form planets quickly in a large collision. This new image seemed to explain the most distant planets, but because all the known examples were very old (usually billions of years old), this theory remained just that – a theory. So far.
A planet is being born
Last year, my colleagues and I spotted a huge planet, still in the process of forming, around a star about 500 light-years from Earth. This star, named AB Aurigae, has become famous in astronomy circles (opens in a new tab) for the beautiful, intricate, spiral disk that surrounds it. The clusters and waves seen on this disk (and others like them) are consistent with what one might see if a gravitational collapse occurred. But so far, there has been no evidence of planet formation.
The disc around AB Aurigae. The planet that is formed is the drop of light at the bottom. (Image: Currie et al. / Nature Astronomy) This newly discovered planet – named AB Aurigae b – is embedded in a thick, swirling halo of dust and gas, between the indicator spirals and the waves that indicate gravitational collapse. The planet is about 93 times farther from its star than the Earth from the sun, far outside the area in which the traditional theory of nucleus accumulation could explain its formation. This discovery thus provides strong evidence for the alternative theory of gravitational collapse. The discovery was made using observations from the Subaru Telescope at Mauna Kea, Hawaii, as well as the Hubble Space Telescope. Powered by energy from the violent, rapid formation process, the planet is hot enough to glow (about 2000 degrees Celsius). It is this glow that gives the planet its presence. At the same time, the swirling gas and dust around the planet formed are illuminated by the blue light of the central star of AB Aurigae.
Bigger and better telescopes
This new discovery provides a crucial piece of the planet formation puzzle, but the case is by no means closed. As our telescopes get bigger and our observation methods become more advanced, we expect to see many more forming planets caught at all stages of their development, as well as fully formed mature planets like Earth. And finally, we can hope to answer the big questions: how did such a strange and diverse range of planetary systems in the galaxy come about, what are the conditions in these new worlds, and how does our little solar system fit into them? This article is republished by The Conversation (opens in a new tab) with Creative Commons permission. Read the original article (opens in a new tab). Follow all of the Expert Voices topics and discussions – and join the discussion – on Facebook and Twitter. The views expressed are those of the author and do not necessarily reflect the views of the publisher.
