It's raining frogs, toads, crabs, fish, rocks and now aliens

Louis rules out a distant terrestrial source for the mysterious particles, because the red rain was concentrated over Kerala for two months despite changes in climate and wind patterns. Could the cells instead be local pollen or fungal spores washed off trees and houses by the rain? Louis says no, because red rain was collected in buckets placed in wide-open spaces. Equally, he says, the red particles can't be pollen or spores from the ground that accumulated in the atmosphere, because the rain would then have been red at the start of a shower; often the colour came later.

Instead, he links the coloured rain to a meteor airburst. During the early hours of 25 July 2001, just hours before the first red rain fell, several people in the Kottayam district heard a loud sonic boom that made their houses rattle. Louis has interviewed some of those who heard it, and concluded that it was too loud to have been an ordinary thunderclap. It's possible that an incoming meteor exploded in the atmosphere.

Louis then takes a large leap and suggests the meteor was a fragment of a comet harbouring microbes from space. He thinks that is the only explanation for the red rain pattern. The meteor flew over Kerala from north to south, he suggests, shedding fragments and alien microbes in the upper atmosphere, before finally exploding over Kottayam district. There, some of the red microbes mixed with rain clouds and fell fairly quickly, while the rest gradually settled into the clouds and fell in rain over the following weeks.

"Yes, it is an extraordinary claim, but I have to report what I observe," says Louis. "We are not able to explain it by assuming a terrestrial object." The red particles look like biological cells, he stresses, but contain no DNA. They could therefore be exotic, alien life forms unknown to science.

Far-fetched? Certainly sounds it. But the idea would undoubtedly have appealed to the late University of Cambridge astronomer Fred Hoyle, champion of the "panspermia" theory. With Chandra Wickramasinghe of Cardiff University, UK, Hoyle developed the idea that life on Earth evolved from microbes that fell to its surface on a comet. In this picture, primitive life forms could be ubiquitous throughout the universe, peppered among the planets and the stars.

Philosophically, panspermia has a certain appeal. It could resolve the genuine puzzle about why life arose on Earth so fast. The solar system began its life some 4.5 billion years ago as a hostile interplanetary war zone, with rocky missiles pelting everything in sight. Around 3.9 billion years ago, the Earth suffered a particularly violent bombardment that pulverised its crust. Yet carbon isotopes in ancient rocks hint that primitive microbes were thriving just 50,000 years later a blink of an eye in evolutionary terms. Panspermia allows off-the-shelf microbes to arrive on the newly hospitable Earth. This waves away the apparent paradox that the Earth is the only place in the entire cosmos where we've found signs of life. From experience, astronomers assume that if something has occurred once in the universe, it's probably occurred many more times we just haven't seen it yet.

Panspermia developed a touch of giggle-factor when Hoyle and Wickramasinghe blamed extraterrestrial viruses for flu epidemics. But it has come back into fashion of late, and proponents argue there's plenty of evidence for it. Experiments have shown that some tough bacteria can survive for years in space, despite the extreme cold and high levels of radiation. Others have proved that some of these bugs could survive the high-speed collisions that they would experience if they slammed into the Earth on a comet.

The idea of primitive microbes flying around the solar system in its early days is not as wild as it seems. "Most of the rocks near the surface of the Earth are shot through with microbial life. It would be a fairly simple thing for a little piece of the crust to be ejected and life survive and land somewhere else," says Walker. On balance, he says, he'd bet that life began here on Earth. But he wouldn't be that surprised if evidence emerged that life started somewhere else and was delivered to Earth by a hunk of space rock.

In 1996, Martian meteorite ALH 84001 caused a furore when some scientists claimed that it harboured fossil bugs. The case was never proved. "But the most interesting information that we gathered from that meteorite was that when the rock was ejected from Mars and travelled to the Earth, the temperature of the interior never exceeded something like 50 C," says Walker. "Plenty of microbes can survive that, especially spores."

All in all, it seems that panspermia could work. Now Louis thinks the red rain of Kerala provides evidence that it actually does. His new report on the subject, which will appear in Astrophysics and Space Science in the next few months, is impressive in its detail, according to Wainwright. "Everything in the paper is done correctly, there's nothing wacky about it," he says. Grady says it is "very, very thorough indeed".

However, if scientists have a favourite quote, it's this one, popularised by Carl Sagan: "extraordinary claims require extraordinary evidence". I'm hearing it a lot in discussions about the red rain of Kerala. Grady thinks Louis and Kumar have jumped to the extraterrestrial conclusion far too quickly. "They seem to prefer the most bizarre explanation they could find," agrees Charles Cockell at the Open University, who studies the microbiology of extreme rocky environments.

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