Structure and Composition of a White Dwarf Planet

Given everything we’re learning about planets around other suns, it’s frustrating that we have so little information about the chemical composition of the rocky planets we’ve found thus far. Now we have a new study, announced at the San Diego meeting of the American Astronomical Society, that offers data on a ‘planet-like body’ whose surface layers are being consumed by the white dwarf SDSSJ1043+0855. Although it’s been known for some time that the star has been devouring rocky material orbiting around it, the new work offers a striking view of the accretion process and the composition of what was once a differentiated body.

At least, that’s the best interpretation of the data taken from the Keck Observatory’s HIRES spectrometer (installed on the 10-meter Keck I instrument) and the Hubble Space Telescope. White dwarf stars are the remains of stars like the Sun — this one was once a few times the Sun’s mass — that have gone through their red giant phase and expelled all their outer material. The ‘planet-like body’ the researchers refer to is likely the remnant of a surviving planet.

To study the chemical composition of such a world, Carl Melis (UC-San Diego) and Patrick Dufour (Université de Montreal) used the spectra of the rocky accretion material as filtered through the atmosphere of the star. The researchers believe that using these methods, they are able to analyze specific layers of the body undergoing accretion. We learn that the object shows large amounts of carbon, combined with smaller amounts of calcium and oxygen.

We may be looking, Melis and Dufour suggest, at calcium carbonate (CaCO 3 ), a mineral widely found in shelled marine organisms here on Earth. As this news release from Keck Observatory points out, incorporating carbon in the surfaces of rocky objects is difficult, which is why the terrestrial planets in our own Solar System are sometimes described as being in a ‘carbon desert.’ The surface being accreted by SDSSJ1043+0855 shows perhaps as much as several hundred times the amount of carbon found on the surface of the Earth.

Is it possible that life played a role in this object’s history? Melis comments:

“…the presence of such high levels of carbon is unique and really needs to be explained. Our choice of calcium-carbonate as a potential carrier of the carbon provides a natural way for it to be locked up in the planet and eventually delivered to the white dwarf star, is entirely consistent with the observations in hand, and of course is suggestive. That’s really the hidden subtext. When people think about finding extra-terrestrial life, they think about Hollywood dramatizations. But the first evidence of life outside of our Solar system will probably come in a much subtler form. More likely than not, it’s going to come as a nuanced signature that may not be immediately recognizable.”

Image (click to enlarge): Artist’s impression of the surface of the massive, planet-like body being devoured by the white dwarf SDSSJ1043+0855. The Keck Observatory and Hubble Space Telescope data (shown in inset) show calcium and carbon, the presence of which can be explained with a model suggesting the surface of the planet may have been encrusted in limestone (calcium carbonate). This material was removed from the surface of the massive rocky body, probably through large-scale collisions, subsequently shredded into a disk of material, and accreted by the white dwarf star (ringed object seen in the planet’s sky). Credit: A. Hara/C. Melis/W. M. Keck Observatory.

But calcium carbonate isn’t always the result of biology, and the current work examines only the accretion materials that have been absorbed by the parent white dwarf. Melis and Dufour would like to look next at surrounding dust before it falls into the star, using the James Webb Space Telescope if possible to confirm whether calcium carbonate is present. This would allow a better estimate of whether the amount of calcium carbonate is consistent with natural processes.

Centauri Dreams’ take: Calcium carbonate or not, it’s striking that using accreted material in the region of a white dwarf and in its atmosphere can help us understand the structure of an exoplanet. We move beyond bulk chemical composition to differentiate between the layers of the body being accreted. That’s a highly useful tool for studying planetary structure.

The presentation is Melis and Dufour, “The Surface of a Limestone-Rich World?” American Astronomical Society 20 June 2016, AAS Meeting #228, id.#201.03 (abstract).