A Formation Mechanism for Pulsar Planets?

by Paul Gilster on February 24, 2014

CSIRO, the Commonwealth Scientific and Industrial Research Organisation in Australia, is announcing the detection of violent events around the pulsar PSR J0738-4042, some 37,000 light years from Earth in the constellation Puppis. This southern hemisphere constellation was originally part of a larger constellation called Argo Navis, depicting the ship made famous by the journey of Jason and the Argonauts. But Argo Navis was divided into three smaller constellations, leaving Puppis (The Stern) as something of a mythological fragment.

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Image: An artist’s impression of an asteroid breaking up. Credit: NASA/JPL-Caltech.

Whatever its origins, Puppis is also home, from our Earthly perspective, to a pulsar around which radiation and sleeting high energy particles are common. Ryan Shannon, a member of the CSIRO research team, has previously examined how an infalling asteroid from a violent disk around a pulsar might affect it, slowing the pulsar’s spin rate and affecting the shape of the radio pulse that we see on Earth. That’s a useful signal, because pulsars flash a radio beam with great regularity, so that any disruptions call attention to themselves. Says Shannon:

“That is exactly what we see in this case. We think the pulsar’s radio beam zaps the asteroid, vaporising it. But the vaporised particles are electrically charged and they slightly alter the process that creates the pulsar’s beam.”

Long term monitoring has shown that the pulse shape of PSR J0738-4042 changes multiple times between 1988 and 2012, with one event that implies a rock of about a billion tonnes interacting with the pulsar. The abstract of the paper on this work summarizes the finding:

The torque, inferred via the derivative of the rotational period, changes abruptly from 2005 September. This change is accompanied by an emergent radio component that drifts with respect to the rest of the pulse. No known intrinsic pulsar processes can explain these timing and radio emission signatures.

And so we get an interesting mechanism for the formation of large objects around a pulsar. Dust and debris formed in the original stellar explosion that gave birth to the pulsar itself could fall back toward the spinning remnant to create a debris disk that, in turn, gives birth to larger objects. Recall that two planet-sized objects were detected in 1992 around the pulsar PSR 1257+12, though not necessarily formed by the same mechanisms (a third planet was detected five years later). We have much to learn about the various ways pulsar planets may form.

The environment that spawns these results has to give us pause. “If a large rocky object can form here,” Ryan Shannon adds, “planets could form around any star.” True enough, though we should add that pulsar planets still seem to be the rarest of the breed. In any case, this detection is remarkable. We’re reaching out to a distance far larger than that between the Earth and the galactic core to track conditions in this system. With so many ongoing projects, we can all too easily become blasé about just what is happening here, but let’s keep that sense of wonder at full throttle. Twenty-five years ago we had no confirmed exoplanets, and now we’re investigating asteroids in a system 37,000 light years from Earth? A golden era of discovery indeed…

The paper is Brook et al., “Evidence of an Asteroid Encountering a Pulsar,” Astrophysical Journal Letters Vol. 780, No. 2 (2014), L31 (abstract).

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Ron S February 24, 2014 at 13:39

“We think the pulsar’s radio beam zaps the asteroid, vaporising it…

“If a large rocky object can form here…planets could form around any star.”

The article is behind a pay wall so perhaps something is missing from this seeming contradiction. If the pulsar keeps breaking apart small objects how can any body form from a debris disk?

Michael February 24, 2014 at 14:24

Similar news article

‘Astronomers See Asteroids Hitting Distant Pulsar’

http://www.sci-news.com/astronomy/science-asteroids-stant-pulsar-01776.html

“One of these rocks seems to have had a mass of about a billion tones.”

It must have been quite a note! I think they meant tonnes. And if it did hit the neutron star it would have emitted as much energy as the sun does in over ten seconds in micro-seconds, we would have noticed the flash in gamma/X rays. It is also possible that the gravity of the asteroid bent the beam, after all it had 37 000 light years to reach us.

andy February 24, 2014 at 16:54

How common are pulsar planets anyway? The PSR B1257+12 system seems to be something of a one-off: the planet around PSR B1620-26 is probably a result of a capture scenario and the “diamond planet” at PSR J1719-1438 is better described as being a very low mass white dwarf remnant rather than a true planet.

If pulsars regularly ended up forming planets then I would have thought that a few more examples would have turned up by now – pulsar timing is an impressively sensitive detection method!

ljk February 25, 2014 at 9:54

How many astronomers are actually looking for pulsar planets these days? They all want Earthlike worlds around Sol type stars. That might have something to do with the lack of pulsar planets among other reasons.

A. A. Jackson February 25, 2014 at 13:25

PSR B1257+12 ‘s 3 ‘planets’ were discovered by Aleksander Wolszczan in 1990.
I think? Tho , if I remember, it took till 2004 to nail the pulsar as having those planets.
Since the 1990 was a real discovery, I think that was the first confirmation of an extra solar planetary system.
There seem to be 4 now (tho I think 4, or D) is still uncertain.
I remember when it was just 3 that some one wanted the IAU to name them…
Wynken, Blynken, and Nod. Which is just about as clever a set of names for planets I have ever heard!

Paul Gilster February 25, 2014 at 16:28

Al, as I understand it, the Wolszczan and Dale Frail paper on the planet detection came out in 1992, though the discovery of PSR B1257+12 itself was in 1990 — whichever year, this was as far as I know the first exoplanet discovery. Wynken, Blynken and Nod — nice!

ljk February 25, 2014 at 16:41

I wonder what ETI who have found our planet from afar but have not picked up our transmissions yet call us? Has to be more interesting than Earth, or at least I hope so.

Or maybe we are just a catalog number or a set of moving coordinates, considering there are more planets than stars in the Milky Way galaxy (400 billion plus).

andy February 26, 2014 at 19:15

It’s also worth noting the curious property that PSR B1257+12 looks remarkably similar to our own inner solar system, in fact it is probably the best known analogue in terms of planetary system architecture to the inner planets.

I remember that when it was discovered, the system was taken as evidence that planetary formation produced similar results to our own system even in wildly different environments, so that systems like our own should be common. 1995 onwards has basically been a series of nasty shocks in that regard.

Aleksandar Volta July 23, 2014 at 17:50

There seems to be some contradictions surrounding the formation of bodies in pulsar environments and subsequent destruction of the same. Needs more research I guess.

Have these asteroids been detected somehow? Do we know they are asteroids?

What if ETI is just playing “Morse code” via pulsars with any recipients in the galaxy? Could be that certain pulsars are used as “phone booths” in a very “desert-like” neighborhood of space?

Has anyone tried to interpret these variations as something else other than natural phenomena?

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