Two UK astronomers may have found an answer to the 25-year-old
mystery of how planets form in the aftermath of a supernova explosion.
Astronomers Dr Jane Greaves, of the University of Cardiff, and Dr
Wayne Holland, of STFC’s UK Astronomy Technology Centre in Edinburgh
presented their work this week at the National Astronomy Meeting at the
University of Hull.
The first planets outside our solar system were only discovered 25
years ago – not around a normal star like our Sun, but instead orbiting a
tiny, super-dense 'neutron star'. These remnants are left over after a
supernova, the titanic explosion of a star many times more massive than
our own.
Such 'planets in the dark' have turned out to be incredibly rare, and astronomers are puzzled over where they come from. The supernova explosion should destroy any pre-existing planets, and so the neutron star needs to capture more raw materials to form its new companions. These after-death planets can be detected because their gravitational pull alters the times of arrival of radio pulses from the neutron star, or 'pulsar', that otherwise pass us by extremely regularly.
Greaves and Holland believe they have found a way for this to happen.
Greaves explains: "We started looking for the raw materials soon after
the pulsar planets were announced. We had one target, the Geminga pulsar
located 800 light years away in the constellation of Gemini.
Astronomers thought they'd found a planet there in 1997, but later
discounted it because of glitches in the timing. So it was much later
when I went through our sparse data and tried to make an image."
The two scientists observed Geminga using the James Clerk Maxwell
Telescope (JCMT), which operates at submillimetre wavelengths, sited on
Hawaii. The light the astronomers detected has a wavelength of about
half a millimetre, is invisible to the human eye, and struggles to get
through the Earth's atmosphere.
Holland, part of the group that built the JCMT camera the team used –
called 'SCUBA' – notes: "What we saw was very faint. To be sure, we
went back to it in 2013 with the new camera our Edinburgh-based team had
built, SCUBA-2, which we also put on JCMT. Combining the two sets of
data helped to ensure we weren't just seeing some faint artefacts."
Both images showed a signal towards the pulsar, plus an arc around
it. Greaves adds: "This seems to be like a bow-wave – Geminga is moving
incredibly fast through our Galaxy, much faster than the speed of sound
in interstellar gas. We think material gets caught up in the bow-wave,
and then some solid particles drift in towards the pulsar."
Her calculations suggest that this trapped interstellar 'grit' adds
up to at least a few times the mass of the Earth. So the raw materials
could be enough to make future planets.
Greaves cautions that more data is still needed to tackle this
quarter of a century old puzzle: "Our image is quite fuzzy, so we've
applied for time on the international Atacama Large Millimetre Array –
ALMA – to get more detail. We're certainly hoping to see this space-grit
orbiting nicely around the pulsar, rather than some distant blob of
Galactic background!"
If ALMA data confirm their new model for Geminga, the team hope to
explore some similar pulsar systems, and contribute to testing ideas of
planet formation by seeing it happen in exotic environments. This will
add weight to the idea that planet birth is commonplace in the universe.
Contact
Jake Gilmore
STFC Media Manager
Further information
The new work appears in: "The Geminga pulsar wind nebula in the mid-infrared and submillimetre", J. S. Greaves and W. S. Holland, Monthly Notices of the Royal Astronomical Society Letters, in press. A preprint of the paper is available here.
UKATC
UKATC Based at the Royal Observatory in Edinburgh and operated by
STFC, the UK Astronomy Technology Centre (UK ATC) is the national centre
for astronomical technology. The UK ATC designs and builds instruments
for many of the world’s major telescopes. It also project manages UK and
international collaborations and its scientists carry out observational
and theoretical research into questions such as the origins of planets
and galaxies. The UK ATC has been at the forefront of previous key
initiatives at the VLT, including the construction of KMOS (K-band
Multi-Object Spectrograph) which enables 24 objects to be observed
simultaneously in infrared light.
Cardiff University School of Physics and Astronomy
