Researchers say aliens may be in our solar system (we're just not looking hard enough to see them)

Our most detailed space searches cannot see objects less than 33 feet wide

Alien artifacts may also be hidden in our oceans or dense forests

Study suggests other civilisations may not have noticed us yet either

The White House this week issued a statement saying there was no existing evidence that aliens had attempted to contact earth or in fact that they even existed.

But how can the Obama administration be so sure?

The question is being asked by scientists, who say that we have not looked closely enough at outer space and that there might be large alien greetings signs within the solar system that we simply are missing.

And they add that our deep-space probes, Nasa's Pioneer and Voyager craft may be equally too small for extraterrestrial civilisations to notice.

The White House was responding to an inquiry as part of the 'We The People' project, which allows members of the public to submit petitions requesting government action.

One petition asked for a formal acknowledgment of 'an extraterrestrial presence engaging the human race'.

Phil Larson, who works on policy and communications for the White House's Office of Science & Technology Policy, replied : 'The U.S. government has no evidence that any life exists outside our planet, or that an extraterrestrial presence has contacted or engaged any member of the human race.'

But two researchers at Penn State university beg to differ, and their paper on the subject has been accepted by Acta Astronautica and posted online on ArXiv.

Jacob Haqq-Misra, of Rock Ethics Institute, and Ravi Kumar Kopparapu, of the Earth and Environmental Systems Institute, say alien spacecraft or probes the size of a racing yacht could be floating around our solar system, and we're blissfully unaware.

They write: 'The vastness of space, combined with our limited searches to date, implies that any remote unpiloted exploratory probes of extraterrestrial origin would likely remain unnoticed.

'Extraterrestrial artifacts may exist in the solar system without our knowledge, simply because we have not yet searched sufficiently. 

Few if any of the attempts would be capable of detecting a one- to ten-metre probe.'

This measurement, which correlates to between three and 33 feet, is a large enough hole to fly an alien probe through and certainly bigger than anything we've sent out into space in a bid to attract extraterrestrial attention.

The Pioneer and Voyager probes sent out by Nasa in the Sixties and Seventies were all smaller than the paper's window for error.

The Pioneer probes had plaques depicting a man and a woman, while the Voyager probes carried 'Golden Records' that included binary information and photographs.

'Searches to date of the solar system are sufficiently incomplete that we cannot rule out the possibility that non-terrestrial artifacts are present and may even be observing us'

Similar-sized alien plaques and records in space would by invisible to us here on Earth.

The researchers go even further, saying that the equivalent of an alien calling card might even be hiding here on Earth.

They write: 'The surface of the Earth is one of the few places in the solar system that has been almost completely examined at a spatial resolution of less than three feet.'

Yet they point to the ocean floor, dense jungles or deep cave as potential hiding places. 

And even in the high-tech 21st century, new species, submarine features and entire rainforest civilisations are being discovered for the first time.

There's even a chance that the probe would just look like a rock. 

The researchers conclude : 'searches to date of the solar system are sufficiently incomplete that we cannot rule out the possibility that non-terrestrial artifacts are present and may even be observing us.

'The completeness of our search for non-terrestrial objects will inevitably increase as we continue to explore the moon, Mars and other nearby regions of space.'