Scientists are studying spacecraft images to find out whether a small part of Comet Ison survived its close encounter with the sun.

The comet at first seemed to have fallen apart as it approached the sun's sizzling surface, but new images showed a streak of light moving away from the sun that some said could indicate it wasn't game over just yet.

"It certainly appears as if there is an object there that is emitting material," said Alan Fitzsimmons, an astronomer at Queens University in Belfast.

The European Space Agency, which had declared Ison's death on Twitter late on Thursday, was backtracking toiday, saying the comet "continues to surprise".

Comet Ison, essentially a dirty snowball from the fringes of the solar system, was first spotted by a Russian telescope in September last year.

Some sky gazers speculated early on that it might become the comet of the century because of its brightness, although expectations dimmed over time.

The comet was two-thirds of a mile wide as it got within one million miles (1.6 million kilometres) of the sun, which in space terms basically means grazing it.

Nasa solar physicist Alex Young on Thursday the comet had been expected to show up in images from the Solar Dynamics Observatory spacecraft but almost four hours later there was "no sign of it whatsoever".

Images from other spacecraft showed a light streak continuing past the sun, but Mr Young said that was most likely a trail of dust continuing in the comet's trajectory.

However, instead of fading, that trail appeared to get brighter today, suggesting that "at least some small fraction of Ison has remained in one piece," US Navy solar researcher Karl Battams wrote on his blog. He cautioned that even if there is a solid nucleus, it may not survive for long.

Two years ago, a smaller comet, Lovejoy, grazed the sun and survived, but fell apart a couple of days later.