High amounts of UV will do it. Single celled organisms would be sterilized, whilst those with a dead skin layer would be relatively fine, though likely subject to higher amounts of mutations and thus skin cancer. You can adjust the dose to keep the whole planet sterile, but perfectly livable for any organism that has developed sunglasses and clothes. If you need a mechanism to explain the UV, go with high amounts of X-rays hitting an earth type atmosphere, resulting in down-conversion by the atmosphere to UV radiation.

If necessary, you can construct the history of your planet so that it develops complex life, that is then killed off by successive gamma ray bursts nearby. https://en.wikipedia.org/wiki/Gamma-ray_burst. You will need more than one, of course, because a single gamma ray burst will only effect one side of the planet.

Additionally, how sterile do you need it? Your story may be able to function with most complex life killed off, and the only life on the planet are single celled organisms around a deep thermal vent.

High amounts of UV will do it. Single celled organisms would be sterilized, whilst those with a dead skin layer would be relatively fine, though likely subject to higher amounts of mutations and thus skin cancer. You can adjust the dose to keep the whole planet sterile, but perfectly livable for any organism that has developed sunglasses and clothes, and they can grow plants within a greenhouse (if you have dosed your planet with very large amounts of UV continuously). If you need a mechanism to explain the UV, go with high amounts of X-rays hitting an earth type atmosphere, resulting in down-conversion by the atmosphere to UV radiation.

If necessary, you can construct the history of your planet so that it develops complex life, that is then killed off by successive gamma ray bursts nearby. https://en.wikipedia.org/wiki/Gamma-ray_burst. You will need more than one, of course, because a single gamma ray burst will only effect one side of the planet.

Additionally, how sterile do you need it? Your story may be able to function with most complex life killed off, and the only life on the planet are single celled organisms around a deep thermal vent.

High amounts of UV will do it. Single celled organisms would be sterilized, whilst those with a dead skin layer would be relatively fine, though likely subject to higher amounts of mutations and thus skin cancer. You can adjust the dose to keep the whole planet sterile, but perfectly livable for any organism that has developed sunglasses and clothes. If you need a mechanism to explain the UV, go with high amounts of X-rays hitting an earth type atmosphere, resulting in down-conversion by the atmosphere to UV radiation.

High amounts of UV will do it. Single celled organisms would be sterilized, whilst those with a dead skin layer would be relatively fine, though likely subject to higher amounts of mutations and thus skin cancer. You can adjust the dose to keep the whole planet sterile, but perfectly livable for any organism that has developed sunglasses and clothes. If you need a mechanism to explain the UV, go with high amounts of X-rays hitting an earth type atmosphere, resulting in down-conversion by the atmosphere to UV radiation.

If necessary, you can construct the history of your planet so that it develops complex life, that is then killed off by successive gamma ray bursts nearby. https://en.wikipedia.org/wiki/Gamma-ray_burst. You will need more than one, of course, because a single gamma ray burst will only effect one side of the planet.

Additionally, how sterile do you need it? Your story may be able to function with most complex life killed off, and the only life on the planet are single celled organisms around a deep thermal vent.