Chandra Detects the First X-rays Coming From Our Kuiper Belt

Carey Lisse (left) and Ralph McNutt Jr. (right)

We are pleased to welcome a pair of distinguished guests to the Chandra blog. Carey Lisse is currently a principal staff scientist at Johns Hopkins University Applied Physics Laboratory (JHU-APL). He has used ACE, Chandra, EUVE, HST, ROSAT, Spitzer, and XMM-Newton as well as numerous ground based telescopes to study the physical properties of many Solar System objects. Ralph McNutt Jr. is a physicist also at JHU-APL. Among his many other positions, he serves as the co-investigator for the PEPSSI instrument aboard New Horizons. He also been the principal investigator on many other spacecraft and experiments designed to explore the Solar System and beyond. Lisse and McNutt are the 1st and 2nd authors of a paper that is the basis for our latest press release, about the surprising Chandra detection of Pluto.

Once the stuff of science fiction when we were kids, the fantastically successful NASA New Horizons mission's flyby of Pluto in July 2015 has transformed our understanding of Pluto from a point-like object into a fascinating world. It has also inspired a plethora of new observations of the system to take advantage of the once-in-a-lifetime flyby event. We ran one of these sets of observations using NASA's Chandra X-ray Observatory. Launched in 1999, Chandra is now one of the established premier observatories of our time - in fact it is one of NASA's four Great Observatories (along with the Hubble Space Telescope, Spitzer Space Telescope, and the Compton Gamma Ray Observatory ).

It was an ambitious idea to observe Pluto in X-rays using Chandra. On the one hand, Pluto was "known" to be venting neutral gas molecules from its atmosphere into interplanetary space, much like a comet does. Since our discovery in 1996 of X-ray emission from comet Hyakutake using the German ROSAT observatory, we have seen X-ray emission from every close approaching comet we've looked at. On the other hand, Pluto is more than three times farther away than the most distant solar system body ever seen to emit X-rays - the planet Saturn.

So we didn't even bother to try and find X-rays from Uranus or Neptune, we skipped right over them and went to look at their more distant cousin, Pluto!

But we did this for a good reason. The way comets emit X-rays is not because they are hot and highly energetic, like all the other X-ray sources we know of in the sky like stars, black holes, neutron stars, and colliding shock waves. Instead comets are very cold, but they boil off gas from their ices when they approach the Sun -- and that's half of what is needed to make X-rays. The other half is the contribution of the biggest and hottest thing in our solar system: the Sun. The Sun not only emits its own X-rays as it boils and seethes and twists magnetic fields on its incendiary surface, it also blows out a stream of high energy ionized plasma – a gas composed of free electrons and free atomic nuclei – from the million degree tenuous corona (or atmosphere) surrounding its surface. This stream, called the solar wind, contains highly charged ions of hydrogen, helium, carbon, nitrogen, oxygen, iron, magnesium, neon, and sulfur, to name the most abundant species. If one of these ions ever gets near another atom with all its electrons, it will rip one or two of them off, emitting X-ray photons in the process.

So it's the Sun that's really doing almost all of the work in making X-rays in the solar system. Comets and planets just help things along by being a source of electron food for the solar wind, or as good reflectors of X-rays emitted from the Sun’s surface.

So back to Pluto. New Horizons carries two instruments, SWAP (“Solar Wind Around Pluto”) and PEPPSI (“Pluto Energetic Particle Spectrometer Investigation”), to measure the flux and energy and direction of the solar wind at Pluto, along with the New Horizons ALICE UV spectrometer to determine the rate of atmospheric gas loss to space. So all the pieces were there to calculate the rate of X-ray production from Pluto via electron transfer. All we needed to do was detect some X-rays!

Enter Chandra. In 2013 we wrote a "fishing expedition" proposal to the Chandra project up at the Smithsonian Astrophysical Observatory, basically describing this once-in-a-lifetime observing possibility, even though Pluto was so far away. They agreed it was worth a gamble, and gave us about 10 hours of time in a small experiment to observe Pluto. We did this on February 24, 2014. To our delight, we found that we had detected two photons at Pluto’s position, where we expected none.

Now the observant reader may not be impressed by two photons from an object, when we find thousands and millions of photons from stars and planets and comets every second in visible and infrared light. But X-ray photons are of such high energy that they are exceedingly rare, and just a handful of photons can be an important detection. Or as we like to say in X-ray astronomy, "every photon is sacred and has a name".

So our February 2014 fishing expedition was rather successful. However, it was performed with New Horizons more than four AU (four times the Earth-Sun distance) away from Pluto. And to a doubting Thomas, those two photons were interesting, but possibly explained away by other sources, like a background object confused with Pluto.

So with hat in hand we asked the Chandra project for more Pluto observing time to coincide as closely as possible with the New Horizons flyby on July 14, 2015. After some careful discussion and haggling, they graciously granted us about another 40 hours of observations, four times the original amount we used in February 2014. And they allowed us to turn Chandra to look at Pluto as soon as it could in its orbit (the highly sensitive Chandra has to avoid looking too close to the Sun), which Pluto was on the July 30, 2015.

We found another six photons at the location of Pluto, which had moved substantially across the sky since 24 Feb 2014, meaning that these X-rays were not from a background source. And we now had enough X-rays to look at their energy spectrum – how the intensity of the radiation varies with its wavelength – and found that it matched what we expected from solar wind electron transfer.

This was great, and meant we could declare success and celebrate. Until a smart member of our New Horizons X-ray team, Randy Gladstone, asked if we're sure there was enough neutral gas and solar wind to make the detected X-rays. Those eight photons collected in about 50 hours, after a careful background removal, equate to ~3x1024 X-ray photons per second released from Pluto. From the New Horizons flyby results we now know Pluto is losing gas at a rate of about 6x1025 molecules per second, providing plenty enough neutral gas atoms, but about 50 times less than "known" before the flyby, hence Randy's concern. The problem is that New Horizons' measurements of the solar wind flux have it at about a factor of 40 too low in providing the carbon, nitrogen, and oxygen ions to steal those electrons and emit X-rays. A factor of 40 too few assuming the solar wind has the same composition and ion abundance as near Earth, and simply flows into Pluto like a wavefront unperturbed by Pluto.

So what does this all mean? It likely means that Pluto significantly perturbed the solar wind, and causes it to wrap around itself and focus into a long downstream tail, where it mixes with Pluto’s escaping atmosphere and makes X-rays. In fact, the New Horizons SWAP instrument has said just that, that they have detected a tail leading away from Pluto that contains ~1024 CH4 (methane) molecules per second getting ionized, which is consistent with what we need to create the X-rays detected by Chandra. They also figure that this tail extends at least 100 times the radius of the planet.

With our new Chandra result, we think the tail's length is more like 1000 times Pluto’s radius!

It also means we have just invented the field of Kuiper belt X-ray astronomy, and that we can expect to see X-rays from other large Kuiper Belt Objects losing their atmosphere.

Cool!