All About Space
Building a planet in the lab
Meet the scientists who are creating exotic worlds
In Rochester, New York, United States, there is a laboratory that is home to the second most powerful laser in the world. This room can produce temperatures comparable to the core of the Earth, all in the name of research. This sanctuary of scientific research is the Laboratory of Laser Energetics (LLE), part of the University of Rochester’s southern campus.
Within the LLE is the pièce de résistance, the OMEGA laser, which scientists are using to manufacture a new world. Within its target chamber, surrounded by 60 lasers, scientists are peering into the interior of gas giant planets such as Jupiter, Saturn and distant exoplanets that are primarily composed of the most abundant and simplest element in the universe: hydrogen.
Hydrogen, made up of one proton and one electron, makes up 74 per cent of the normal matter in our universe, with second place going to helium, which which makes up 24 per cent. These percentages are similar to our Sun, but the Jovian planets beyond the asteroid belt are also known to be made up of mostly hydrogen. Jupiter and Saturn both consist of about 90 per cent hydrogen, with the heavier elements sinking towards their respective cores and hydrogen dominating the residual layers, spanning tens of thousands of kilometres in radius.
On the face of each planet the layer of hydrogen that amateur astronomers can observe with a good telescope is hydrogen in its gaseous state, also known as molecular hydrogen (H2). Molecular hydrogen is hydrogen in its most stable state and exists in a diatomic form; this is the state that scientists are most familiar with on Earth.
When you make your way towards the centre of a gas giant such as Jupiter, hydrogen turns into a more exotic state – metallic hydrogen. This is the state of hydrogen scientists are trying to create at the LLE, as Dr Mohamed Zaghoo, a research associate at the LLE, tells All About Space:
“This is indeed a very exciting area of research where laboratory data and space observation are equally valuable to build a more accurate picture of hydrogen-rich planets.”
Much like how water changes state based on temperature, melting ice into liquid water and further evaporating into water vapour with a continual increase in temperature, gaseous hydrogen will morph into its metallic hydrogen state with an increase in temperature and pressure. These conditions provide enough energy to pull the electrons away from their protons and form a sea of protons, which can also be thought of as hydrogen ions, and freeflowing electrons. As there are many free electrons shifting through this state of matter it is thought to be superconductive.
Metallic hydrogen is theorised and heavily supported – but not yet proven – to be the condition present in the inner layers of Jupiter and Saturn. It is also thought that by learning more about the metallic hydrogen state we can learn about a gas giant’s magnetosphere and dynamo effect. A magnetosphere is the area occupied by a planet’s magnetic field and the dynamo effect is the churning of conductive internal material powering the magnetosphere. Due to metallic hydrogen’s superconductive nature, it would explain why Jupiter has the most powerful magnetosphere.
“Laboratory data and space observation are equally valuable to build a more accurate picture of hydrogen-rich planets”
Metallic hydrogen is not easy to create, however. Pressure needs to be raised to between 1.4 and 1.7 megabar, and temperatures between 1,500 and 2,400 degrees Celsius (2,700 and 4,400 degrees Fahrenheit). This is over a thousand-times the pressure of Earth’s average atmospheric pressure and these temperatures are capable of melting lead. These unearthly conditions are extremely difficult to manufacture, hence why this research is being conducted in one of the world’s most unique laboratories.
In early 2017, Professor of Natural Sciences Isaac Silvera and postdoctoral fellow Dr Ranga Dias, both of Harvard University in Cambridge, Massachusetts, were actually able to create metallic hydrogen using a nifty but expensive piece of equipment called a diamond anvil cell (DAC). This is what was fitted at the centre of OMEGA’s target chamber containing the hydrogen sample. “The diamond anvil cells generate the pressures statically, while the lasers [OMEGA] generate the temperatures,” explains Zaghoo.
A DAC is a high-pressure device that has two opposing diamonds fixed together via rhenium gaskets. The diamond tips are polished to ensure there will be minimal cracks or damage that will lessen the pressure within the central sample region, which measures less than a millimetre. It is incredible to think that in a laboratory that stands ten metres (32 feet) tall and is approximately 100 metres (328 feet) in length, the culmination of the scientists’ work all comes from a sample that is unperceivable to the human eye.
This sample is fitted within the DAC, which is approximately the size of a coke can, and then fitted inside the OMEGA target chamber. In the same way that people stamp on a coke can to squash it down, scientists and engineers have to squeeze the diamonds together and ramp up the pressure with each millimetre of compression. When this apparatus is fixed into position and the sample is under pressures comparable to the interior of Jupiter, researchers fire the lasers.
As the experiment room can rise to uncomfortable temperatures the scientists are situated in the LLE’s control room when the heating begins. The OMEGA laser drivers are fired up, initiating the process by creating shaped seed pulses via a 60-beam ultraviolet neodymium glass laser, which are intensified with stage A amplifiers. By the time the lasers are concentrated on the target chamber OMEGA is capable of producing 30 kilojoules of energy and 60 terawatts of power in just one-billionth of a second. When compared with the fact that a standard oven can use up to 5,000 watts, the OMEGA produces 12 billion-times more energy in a fraction of the time.
Observations are the most essential part of any experiment, but it is impossible to observe any change with the naked eye. Because of this, researchers such as Zaghoo use an ingenious datacollection method. “The data is collected using fast optical detectors that record the reflectance of a laser light off the hydrogen samples. The premise of the measurements is simple: metallic substances reflect light, while insulating ones don’t. Thus we measure how much light the compressed and hot hydrogen reflects,” explains Zaghoo. “At low pressures and temperatures hydrogen is transparent liquid, but at sufficiently high pressures the molecular form of hydrogen breaks down under
“At sufficiently high pressures the molecular form of hydrogen breaks down and gives away its electrons”
the crushing pressure and gives away its electrons, enabling conduction and light reflection.”
When the light is reflected back into their detectors scientists are overjoyed, yet there is still work to do. They have had a glimpse into the interior of gas giant planets – and in particular to this study, Jupiter – and after decoding the data they can uncover the mystery of their intense magnetospheres and dynamos. “By varying the final pressures and temperatures we were able to build this ‘conductivity profile’ or map of metallic hydrogen conductivity at different depths along Jupiter’s interior,” says Zaghoo. The conductivity map created in this instance has showed that Jupiter’s dynamo originates closer to the surface than Earth’s dynamo.
The research doesn’t end there though. In science collaboration is key, and there are different missions with different perspectives that need to be accounted for. By incorporating these results into simulations that also include up-close observations from spacecraft like NASA’s Juno mission – the space probe currently in orbit around Jupiter, gathering vital information about its magnetosphere and cloud top composition – scientists are continuing to understand the inner workings of Jupiter to a level that is simply incredible.
“One of the great insights that Juno very recently provided is that the thunderous winds, or bands, that characteristically distinguish the planet’s surface extend much deeper into the planets interior, to almost 3,000 kilometres (1,900 miles) in depth,” says Zaghoo. “By combining this data with our experimental conductivity profile we are able to further constrain the depth of the dynamo process and the interaction between the strong swirling winds with the conductive fluid deep inside.”
Dr Mohamed Zaghoo