The planets that flank Earth huddle close together in the dawn sky over the next few days. Venus and Mars are quite low in the east-southeast during the waxing twilight, so you need a clear horizon to spot them. And binoculars wouldn’t hurt.
Venus is the second planet out from the Sun, Earth is third, and Mars is fourth. Their orbits are separated by tens of millions of miles. Yet all three worlds are inside the Sun’s “habitable zone.” That’s the distance from the Sun where the temperature is just right for liquid water on the surface of a planet — or could be.
Venus is far too hot for liquid water. And Mars is too cold. When the planets formed, however, conditions could have been different. There’s evidence that oceans could have covered much of the surfaces of both planets.
Over the eons, though, conditions changed. Venus’s thick atmosphere trapped heat from the Sun, making the planet hotter. It got hot enough to bake out gases from the rocks, making Venus even hotter. Mars, on the other hand, lost much of its atmosphere. Today, the air is much too thin for liquid water to last for more than a few minutes. That leaves only Earth as a comfortable home for life — in the middle of the habitable zone.
Venus is the “morning star.” Fainter Mars is below it tomorrow, and closer on Wednesday and Thursday. After that, they’ll separate, with Mars climbing a little higher each day, and Venus dropping into the Sun’s glare.