Where's best to look if you're trying to find signs of alien life?
It's a question that space experts have been arguing about for decades.
Astro-biologists reckon the most likely place to find life is on the icy moons orbiting Saturn and Jupiter.
But American space agency, Nasa, spends more time and money exploring Mars.
The agency's Curiosity Rover recently discovered signs of what could have been rivers on the red planet.
That got space scientists excited because water is needed to support life as we know it.
But others reckon we should be concentrating the search for alien life further afield, looking beyond the planets closest to us.
Martin's been looking a little closer at the search for aliens - check out his video above!
And for more on the issue, watch Joe's report from Newsround's Space Week!