Free Astronomy Magazine July-August 2016

SPACE CHRONICLES “We've known for some time that examining the accreted remains of rocky planets in the atmosphere of their host white dwarf star can give bulk chemical composition informa- tion, and now it looks like we can even hone in on specific layers of an accreted body in some fortu- itous cases,” Melis said. Determining the chemical com- position or struc- ture of planets outside of the Solar system to date has been elusive at best. “It’s a huge issue in exo-planetol- ogy right now,” Melis added. “The major exo- planet identify- ing methods can't tell you what a planet is made of or what it's struc- ture is.” While the finding will provide a new angle for scientists to study the chem- ical composition and structure of rocky planets, the possibility that life may have contributed to the infer- red mineralogy certainly intrigued the team. The researchers’ finding shows that SDSSJ1043+0855 is accreting the sur- face of a body that has large en- hancements of carbon. This feature — combined with mild enhance- ments of calcium and oxygen — points to the possibility of the mate- rial coming in the form of calcium- carbonate, a mineral that is often associated with shelled marine or- ganisms here on Earth. Calcium-car- bonate is attractive as a mineral constituent of this planet-like body as incorporating and entraining car- bon in rocky objects (especially their surfaces) is difficult. The terrestrial planets in our Solar system are said to live in a “carbon desert” since they are so heavily depleted in this element — the planetary surface being accreted by this white dwarf star could have as much as several hundred times more carbon than the surface of the Earth. “This method allows us to truly get a glimpse of what aliens might be standing on,” Melis said. “In this particular case, 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 pro- vides a natural way for it to be lock- ed up in the planet and eventually delivered to the white dwarf star, is entirely consistent with the observa- tions in hand, and of course is sug- gestive. When people think about finding extra-terrestrial life, they think about Hollywood dramatiza- tions. But the first evidence of life outside of our Solar system will prob- ably come in a much subtler form. More likely than not, it’s going to come as a nuanced signature that may not be immediately recogniz- able.” Non-biological processes can pro- duce calcium-carbonate too, so its presence isn't nec- essarily a smoking gun, even if it is confirmed. “There’s a lot of hoops to jump through before we can settle on the conclusion that life was in- volved in what we are observing,” Dufour said. Spe- cifically, the infer- red presence of calcium-carbon- ate came from ex- amining the atom- ic leftovers of the planet accretion event in the at- mosphere of the white dwarf star – after the pre- sumed dust from the planet’s demol- ished surface was consumed by the white dwarf. The next step will be to look at the dust in a mineral state before it falls into the star, to both confirm its composition and to mea- sure its concentration. “Future obser- vations with the James Webb Space Telescope can confirm calcium-car- bonate if it is present. If we are able to get to that point, then you have to ask: Is there enough there for it to be produced with natural processes?” Melis concluded. While the presence of the calcium-carbonate is still in question, the study shows strong evi- dence that the accreted material is al- most certainly coming from the outer layers of a planet-like object and that white dwarf stars hold promise in in- forming on the structure of planets outside of the Solar system. A rtist's impression of the surface of the massive, planet-like body being devour- ed 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, sub- sequently shredded into a disk of material, and accreted by the white dwarf star (ringed object seen in the planet's sky). [A. Hara/C. Melis/W. M. Keck Observatory] n