Ramp compression of iron provides insight into core conditions of large rocky exoplanets

April 17, 2018 by Breanna Bishop, Lawrence Livermore National Laboratory
High-power lasers at the National Ignition Facility are focused onto a multi-stepped iron sample at the center of the 10-meter-diameter target chamber. These experiments measure the equation of state of iron under core conditions of large rocky exoplanets. Credit: Lawrence Livermore National Laboratory

In a paper published today by Nature Astronomy, a team of researchers from Lawrence Livermore National Laboratory (LLNL), Princeton University, Johns Hopkins University and the University of Rochester have provided the first experimentally based mass-radius relationship for a hypothetical pure iron planet at super-Earth core conditions.

This discovery can be used to evaluate plausible compositional space for large, rocky exoplanets, forming the basis of future planetary interior models, which in turn can be used to more accurately interpret observation data from the Kepler space mission and aid in identifying planets suitable for habitability.

"The discovery of large numbers of planets outside our solar system has been one of the most exciting scientific discoveries of this generation," said Ray Smith, a physicist at LLNL and lead author of the research. "These discoveries raise fundamental questions. What are the different types of and how do they form and evolve? Which of these objects can potentially sustain surface conditions suitable for life? To address such questions, it is necessary to understand the composition and of these objects."

Of the more than 4,000 confirmed and candidate extrasolar planets, those that are one to four times the radius of the Earth are now known to be the most abundant. This size range, which spans between Earth and Neptune, is not represented in our own solar system, indicating that planets form over a wider range of physical conditions than previously thought.

"Determining the interior structure and composition of these super-Earth is challenging but is crucial to understanding the diversity and evolution of planetary systems within our galaxy," Smith said.

As core pressures for even a 5×-Earth-mass planet can reach as high as 2 million atmospheres, a fundamental requirement for constraining exoplanetary composition and interior structure is an accurate determination of the material properties at . Iron (Fe) is a cosmochemically abundant element and, as the dominant constituent of terrestrial planetary cores, is a key material for studying super-Earth interiors. A detailed understanding of the properties of iron at super-Earth conditions is an essential component of the team's experiments.

The researchers describe a new generation of high-power laser experiments, which use ramp compression techniques to provide the first absolute equation of state measurements of Fe at the extreme and density conditions found within super-Earth cores. Such shock-free dynamic compression is uniquely suited for compressing matter with minimal heating to TPa pressures (1 TPa = 10 million atmospheres).

The experiments were conducted at the LLNL's National Ignition Facility (NIF). NIF, the world's largest and most energetic laser, can deliver up to 2 megajoules of laser energy over 30 nanoseconds and provides the necessary laser power and control to ramp compress materials to TPa pressures. The team's experiments reached peak pressures of 1.4 TPa, four times higher pressure than previous static results, representing core conditions found with a 3-4x Earth mass planet.

"Planetary interior models, which rely on a description of constituent materials under extreme pressures, are commonly based on extrapolations of low-pressure data and produce a wide range of predicated material states. Our experimental data provides a firmer basis for establishing the properties of a super-Earth planet with a pure iron planet," Smith said. "Furthermore, our study demonstrates the capability for determination of equations of state and other key thermodynamic properties of planetary core materials at pressures well beyond those of conventional static techniques. Such information is crucial for advancing our understanding of the structure and dynamics of large rocky exoplanets and their evolution."

Future experiments on NIF will extend the study of planetary to several TPa while combining nanosecond X-ray diffraction techniques to determine the crystal structure evolution with pressure.

Explore further: Under pressure: Ramp-compression smashes record

More information: Raymond F. Smith et al. Equation of state of iron under core conditions of large rocky exoplanets, Nature Astronomy (2018). DOI: 10.1038/s41550-018-0437-9

Related Stories

Under pressure: Ramp-compression smashes record

November 11, 2011

In the first university-based planetary science experiment at the National Ignition Facility (NIF), researchers have gradually compressed a diamond sample to a record pressure of 50 megabars (50 million times Earth's atmospheric ...

Probing the possibility of life on super-Earths

June 19, 2017

Along with its aesthetic function of helping create the glorious Aurora Borealis, or Northern Lights, the powerful magnetic field surrounding our planet has a fairly important practical value as well: It makes life possible.

A stellar system with three super-Earths

March 2, 2018

Over 3500 extra-solar planets have been confirmed to date. Most of them were discovered using the transit method, and astronomers can combine the transit light curves with velocity wobble observations to determine the planet's ...

Recommended for you

Supersharp images from new VLT adaptive optics

July 18, 2018

ESO's Very Large Telescope (VLT) has achieved first light with a new adaptive optics mode called laser tomography—and has captured remarkably sharp test images of the planet Neptune and other objects. The MUSE instrument ...

Jupiter's moon count reaches 79, including tiny 'oddball'

July 17, 2018

Twelve new moons orbiting Jupiter have been found—11 "normal" outer moons, and one that they're calling an "oddball." This brings Jupiter's total number of known moons to a whopping 79—the most of any planet in our Solar ...

Astronomers find a famous exoplanet's doppelgänger

July 17, 2018

When it comes to extrasolar planets, appearances can be deceiving. Astronomers have imaged a new planet, and it appears nearly identical to one of the best studied gas-giant planets. But this doppelgänger differs in one ...

Dawn mission to gather more data in home stretch

July 17, 2018

As NASA's Dawn spacecraft prepares to wrap up its groundbreaking 11-year mission, which has included two successful extended missions at Ceres, it will continue to explore—collecting images and other data.

Brown dwarf detected in the CoRoT-20 system

July 16, 2018

An international group of astronomers has discovered a new substellar object in the planetary system CoRoT-20. The newly identified object was classified as a brown dwarf due to its mass, which is greater than that of the ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.