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Taking a picture of a far-off world聽may not be enough to identify what sort of planet it is. We know this because from outside our solar system, every other planet except for Jupiter could be mistaken for Earth at some angles.

When planets beyond our solar system, called exoplanets, are discovered via directly taking an image of them, we usually only get two pieces of information from that image: the planet鈥檚 apparent brightness and its apparent distance from its star.

and at Cornell University in New York investigated a phenomenon called planet confusion 鈥� asking whether, given only those two pieces of information, we are likely to confuse one planet for another. They used our solar system as an example, calculating how often different planets could have properties that would make us think they were Earth from afar.

鈥淚ntuitively, you would think planets that appear brighter are bigger, and stars that appear farther from their star are farther from their star 鈥� but that鈥檚 not quite right,鈥� says at McGill University in Montreal, Canada. In fact, the planet鈥檚 apparent brightness is related to both its size and its reflectivity, and its apparent location is related to both its actual distance from its star and its position on its orbit.

The researchers found that from about 72 per cent of viewing angles, there are locations in Venus鈥檚 orbit where it could be mistaken for Earth. Mars and Mercury are the next most likely to impersonate our planet 鈥� respectively, about 43 and 36 per cent of the angles from which you could view them present an opportunity for confusion. Saturn, Uranus and Neptune can only look like Earth from less than 4 per cent of locations, and Jupiter is so colossal that it never looks like Earth no matter how you look at it.

Making such a mix-up could be a costly mistake. 鈥淭he rough cost for a 1.5-day observation of an exoplanet is about $2.4 million,鈥� says Keithly. 鈥淪o if we make a detection and then we want to follow up on it because it seems Earth-like, we could be wasting $2.4 million and time that we could be using to find other exoplanets.鈥�

Luckily, while there are many viewing angles from which it is possible that another of our solar system鈥檚 planets will look identical to Earth, the probability of that actually happening is relatively low. 鈥淎lthough it鈥檚 true that Neptune or Saturn or whatever can masquerade as the Earth if you catch it at the right place in its orbit, it鈥檚 actually really unlikely,鈥� says Cowan. 鈥淎 broken clock is right twice a day, but you鈥檇 have to get really unlucky to look at a broken clock and have it tell you the right time.鈥�

This also isn鈥檛 an intractable problem 鈥� if we make multiple observations of an exoplanet over time, its motion will reveal its true distance from its star. Other types of observations, like examining the colour of the light bouncing off the planet, can help distinguish what kind of world it is too. Still, when we are looking for Earth-like worlds in the distant universe, we have to be careful to make sure the exoplanets we spot really are as they appear.

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