"Yes, Jupiter, for instance, is very much larger than the moon and the earth; and Professor Proctor tells us it will take Jupiter millions of years to become as cool as the earth, while the moon was as cool as the earth millions of years ago. Here is a picture of the planet; but its surface is changing so constantly, that it seldom appears the same on two nights in succession. Jupiter at present is wrapped in enormous volumes of thin cloud that rises up from a melted and boiling mass in the centre. Professor Newcomb supposes that there is only a comparatively small core of liquid, the greater part of the planet being made up of seething vapor. So you see it would be about as difficult to live on Jupiter as in a steam-boiler, or a caldron of molten lead. Since last summer a great red spot has been noticed on the surface of the planet, which has attracted much attention. Some think it is an immense opening, large enough for our earth to be dropped through."
"Are the other planets such dreadful places?" asked May.
"Saturn seems to be in about the same condition as Jupiter. Mars is thought to be solid, and to have land, water, and air. It has also two brilliant white spots on opposite sides, which are supposed to be vast fields of ice and snow. But the water seems to be disappearing; and the time when the planet could be inhabited is thought to be long gone by."
"Where does the water go?" asked Joe.
"Probably it sinks into the cracks or fissures which form in the crust of the planet when it begins to shrivel up with the cold."
"Then it must be like a great frozen grave-yard," said May. "But is there no other planet that is pleasanter to think about?"
"The one that seems on the whole to be most like our own is Venus, and so Professor Proctor calls it our sister planet. It is so close to the sun that it is hidden most of the time, being only seen for a while before sunrise, and at other times a while after sunset. In the one case it is called the morning, and in the other the evening star. Also there is Mercury, still nearer the sun, and hidden almost all the time."
"Then," said May, "there seems to be no way of knowing anything about there being people like us in other worlds; and the more we look into it, the more uncertain we become."
"That is about the way the case stands," said the Professor. "But if science continues to make as rapid progress as it has lately done, we may hope that it will yet throw more light on the question."
"How many planets are there?" asked Joe.