Like eggs and snowflakes, comets are fragile and subject to breaking. When one swings too close to the our central star, the sun’s combined gravity and searing heat can cause a comet to crumble to pieces. The pieces can further disintegrate into an ever-expanding cloud of gas, water vapor and dust and disperse along the comet’s orbit. Comets are friable or liable to fall apart to begin with, so sometimes it takes only a “nudge,” so to speak, to get the process of destruction rolling. Astronomers have tracked lots of these fizzled comets. Most pass by unnoticed except by the vigilant eye of the Solar and Heliospheric Observatory (SOHO), which records regular flame-outs of small comets. But occasionally a famous once falls to pieces as Comet ISON did while attempting to round the sun in December 2013.
Recently, scientists using the Hubble Space Telescope have observed, for the first time, a massive, comet-like object that was ripped apart and scattered in the atmosphere of a white dwarf. The destroyed object had a chemical composition similar to Halley’s Comet but was 100,000 times more massive than its famous counterpart. In this cosmic version of the David vs Goliath story, the big comet didn’t stand a chance against the tiny but gravitationally dominant star.
The white dwarf, named WD 1425+540, is a highly-compressed, planet-sized star about 170 light-years from Earth in the constellation of Bootes the Herdsman. Most white dwarfs squeeze a sun’s worth of mass into an Earth-sized orb, so that any object that passes near them really feels the wrath of gravity. That same gravity also pulls down on a dwarf’s atmosphere; only the airiest of elements make up a white dwarf’s “air.” Anything heavier gets dragged down to the surface by gravity.
While studying its atmosphere using both the Hubble Space Telescope and the 10-meter (394-inch) Keck Telescope near the summit of Mauna Kea in Hawaii, a research team found evidence for comet-stuff “polluting” the star’s otherwise pure hydrogen-helium atmosphere. Carbon, oxygen (as found in water – H2O), sulphur and even nitrogen, all materials common to comets, were floating around up there.
While astronomers have detected debris from sundered asteroids in more than a dozen white dwarfs’ atmospheres, this is the first time remnants of icy, comet-like material have been found including. Scientists were most excited by the nitrogen finding — the amount nearly matched that seen in Halley’s Comet and the first detection of nitrogen in the debris falling on a white dwarf.
“Nitrogen is a very important element for life as we know it. This particular object is quite rich in nitrogen, more so than any object observed in our solar system,” said Lead author Siyi Xu of the European Southern Observatory. Comets and other distant icy bodies like Pluto, its largest moon Charon and the dwarf planet Eris in the distant Kuiper Belt beyond Neptune are also rich in nitrogen. It shows up as ice on their surfaces because of the extreme cold so far from the sun.
Astronomers think the giant comet originally orbited the dwarf’s version of the Kuiper Belt. Perhaps its companion star, an orange dwarf, or undiscovered planets gave the comet a gravitational kick, sending it on a collision course with the host star. The dwarf’s fierce gravity and heat broke it to bits and digested the pieces, leaving only atmospheric traces of what must have been a magnificent object. What a way to go!
Based on the elements they saw “polluting” the dwarf’s atmosphere, the team determined that the object had a chemical composition similar to the famous Halley’s Comet in our own solar system but was 100,000 times more massive and had twice as much water as its counterpart.
Studying the WD 1425+540 system, we see that icy bodies are also present in other planetary systems and could have seeded growing exoplanets with the water, carbon, nitrogen and sulfur needed to prepare the groundwork for life. For more on the topic, you can read the scientific paper.