Imagine you could travel at the speed of light, 300,000km/s. If you set off from Earth today, then you would reach the Oort CloudA theoretical cloud of tiny, icy planets surrounding the entire solar system., at the edge of the solar system, in about a year and 10 months. After four years and three months, you would arrive at the closest star to the Sun, Proxima CentauriA small star in the constellation Centaurus. Earlier this year it was thought that a signal might be emitting from the star, but it was found to be interference from human technology..

If you kept going, it would take about 950,000 years - three times as long as humans have existed on Earth - to hit the edge of our galaxy, the Milky WayThe galaxy that contains our solar system. It has the structure of a disc, but looks to us like a band because we are inside it.. And if you were patient, after some 28 million years, you would find yourself in a whole different galaxy, on a Saturn-sized planet called M51-ULS-1, which yesterday became the first planet ever to be discovered outside our galaxy.

The new discovery was made using transits, a common method for detecting exoplanetsThe name given to any planets that exist outside the solar system.. With this technique, scientists monitor the light emitted from a star. Each time it orbits, a planet temporarily blocks the light from reaching the Earth. If there are regular, uniform dips in light intensity, then it is likely a planet is there.

The transit method has already been used to discover 3,428 planets. But it has not generally been useful for discovering planets in other galaxies, because the changes in light intensity are tiny. A large planet, around the size of Jupiter, will only cause a drop of about 1%, while an Earth-size planet will cause a decrease of just 0.001%.

That is why scientists working with Nasa's Chandra TelescopeA space telescope in orbit around the Earth that picks up X-rays.   tried a new spin on this old trick: measuring X-raysA form of high-energy electromagnetic radiation. They are emitted by extremely hot gases.. M51-ULS-1 revolves around either a neutron star or a black hole. Both these phenomena superheat the material near them, which causes it to release X-rays. As it orbits, the planet temporarily blocks these X-rays, allowing it to be detected and measured in the same way.

This new technique could revolutionise the search for exoplanets, but it also leaves room for doubt. Some scientists have suggested that the new finding is not proof of a new planet at all: the drop in X-ray intensity could have been caused by a cloud of gas and dust instead.

If it is a planet, it will not complete a full orbit for another 70 years, so there is no way of verifying the data until then.

But if planets similar to those in our galaxy do exist elsewhere, then the chances that we are not alone in the universe increase dramatically.

Astronomers often make reference to the Drake EquationAn equation for calculating the likelihood of coming into contact with alien life. Most of the figures we need to make a precise calculation are not currently available. It is named after its creator, astronomer Frank Drake., which is used to calculate the likelihood of intelligent alien life. It takes into account the rate of star formation, the conditions needed for life to flourish, and, importantly, the number of planetary systems there are in total.

So if there are many detectable planets outside the Milky Way, then our chances of running into aliens are much higher.

However, communicating with these civilisations is currently impossible. We do not have any means of communication that can travel faster than light, so a conversation with an alien society on M51-ULS-1 would have gaps of at least 28 million years.

Yes, say some. The chances of a planet developing intelligent life are small, but if there are trillions of planets, it becomes much likelier that some of them experienced the same mind-boggling coincidences that produced human beings on Earth.

Not at all, say others. Maybe alien civilisations did exist at some point, or will exist in future. But the emergence of intelligent life on Earth took 13.7 billion years to happen, and it might not last beyond the next century. We should not expect alien life to have evolved at the same time as us.