"Brazen! It is brazen!" Prof Paul Sereno says down the phone line from Chicago.

He makes no effort to disguise his anger that a rare meteorite from Mars discovered two years ago in the West African nation of Niger ended up being auctioned off in New York last month to an unnamed buyer.

The palaeontologist, who has close connections with the country, believes it should be back in Niger.

This millions-of-years-old piece of the Red Planet, the largest ever found on Earth, fetched $4.3m (£3.2m) at Sotheby's. Like the buyer, the seller was kept anonymous.

But it is unclear if any of this money went to Niger.

Fragments of extraterrestrial material that have made their way to Earth have long inspired reverence among humans – some ending up as religious objects, others as curiosities for display. More recently, many have become the subject of scientific study.

The trade in meteorites has been compared to the art market, with aesthetics and rarity affecting the price.

At first, there was a sense of awe surrounding the public display of this extraordinary Martian find – less than 400 of the 50,000 meteorites discovered have been shown to come from our planetary neighbour.

The photographs taken at Sotheby's of the 24.7kg (54lb) rock – appearing in the lights to glow silver and red – compounded this feeling.

But then some people started asking questions about how it ended up under the auctioneer's hammer.

Not least the government of Niger itself, which, in a statement, "expressed doubts about the legality of its export, raising concerns about possible illicit international trafficking".

Sotheby's strongly disputes this, saying the correct procedures were followed, but Niger has now launched an investigation into the circumstances of the discovery and sale of the meteorite, which has been given the scientific and unromantic name NWA 16788 (NWA standing for north-west Africa).

Little has been made public about how it ended up at a world-renowned auction house in the US.

An Italian academic article published last year said that it was found on 16 November 2023 in the Sahara Desert in Niger's Agadez region, 90km (56 miles) to the west of the Chirfa Oasis, by "a meteorite hunter, whose identity remained undisclosed".

Meteorites can fall anywhere on Earth, but because of the favourable climate for preservation and the lack of human disturbance, the Sahara has become a prime spot for their discovery. People scour the inhospitable landscape stretching across several countries in the hope of finding one to sell on.

According to the Italian article, NWA 16788, was "sold by the local community to an international dealer" and was then transferred to a private gallery in the Italian city of Arezzo.

The University of Florence's magazine described the person as "an important Italian gallery owner".

A team of scientists led by Giovanni Pratesi, mineralogy professor at the university, was able to examine it to learn more about its structure and where it came from. The meteorite was then briefly on display last year in Italy, including at the Italian Space Agency in Rome.

It was next seen in public in New York last month, minus two slices that stayed in Italy for more research.

Sotheby's said that NWA 16788 was "exported from Niger and transported in line with all relevant international procedures.

"As with everything we sell, all relevant documentation was in order at each stage of its journey, in accordance with best practice and the requirements of the countries involved."

A spokesperson added that Sotheby's was aware of reports that Niger is investigating the export of the meteorite and "we are reviewing the information available to us in light of the question raised".

Prof Sereno, who founded the organisation NigerHeritage a decade ago, is convinced Nigerien law was broken.

The academic with the University of Chicago, who has spent years uncovering the country's vast deposits of dinosaur bones in the Sahara, campaigns to get Niger's cultural and natural heritage – including anything that has fallen from outer space - returned.

A stunning museum on an island on the River Niger that runs through the capital, Niamey, is being planned to house these artefacts.

"International law says you cannot simply take something that is important to the heritage of a country - be it a cultural item, a physical item, a natural item, an extraterrestrial item - out of the country. You know we've moved on from colonial times when all this was okay," Prof Sereno says.

A series of global agreements, including under the UN's cultural organisation Unesco, have tried to regulate the trade in these objects. But, according to a 2019 study by international law expert Max Gounelle, when it comes to meteorites, while they could be included, there remains some ambiguity about whether they are covered by these agreements. It is left to individual states to clarify the position.

Niger passed its own law in 1997 aimed at protecting its heritage.

Prof Sereno points to one section with a detailed list of all the categories included. "Mineralogical specimens" are mentioned among the art works, architecture and archaeological finds but meteorites are not specifically named.

In its statement on the Sotheby's sale, Niger admitted that it "does not yet have specific legislation on meteorites" - a line that the auction house also pointed out. But it remains unclear how someone was able to get such a heavy, conspicuous artefact out of the country without the authorities apparently noticing.

Morocco has faced a similar issue with the huge number of meteorites - more than 1,000 - found within its borders, which include a part of the Sahara.

More than two decades ago the country experienced what author Helen Gordon described as a "Saharan gold rush", fuelled in part by laxer regulations and a more stable political environment than some of its neighbours.

In her recent book The Meteorites, she wrote that Morocco was "one of the world's greatest exporters of space rocks".

Prof Hasnaa Chennaoui Aoudjehane has spent much of the past 25 years trying to hold on to some of that extraterrestrial material for her country.

"It's a part of us, it's a part of our heritage… it's part of our identity and it's important to be proud of the richness of the country," the geologist tells the BBC.

The professor is not against the trade in meteorites but has been instrumental in the introduction of measures aimed at regulating the business. She admits though that the new rules have not been entirely successful in stemming the flow of the meteorites.

In 2011, Prof Chennaoui was responsible for gathering material in the desert from an observed meteorite fall that turned out to be from Mars.

Later named the Tissint meteorite, it weighed 7kg in all, but now she says only 30g remain in Morocco. Some of the rest is in museums around the world, with the biggest piece on display in London's Natural History Museum.

Reflecting on the fate of Niger's Martian meteorite, she says she was not surprised as it is "something that I'm living with for 25 years. It's a pity, we cannot be happy with this, but it's the same state in all our countries."

Prof Sereno hopes that the Sotheby's sale will prove a turning-point - firstly by motivating the Nigerien authorities to act and secondly "if it ever sees the light of day in a public museum, [the museum] is going to have to deal with the fact that Niger is openly contesting it".