Ask Dr. SETI ®
by Dr. John Boardman
From the April 2017 issue of DAGON,
used by permission
Are there other habitable planets out there? And, if there are, can we get to any of them, and how long is it likely to take? The answers to those questions have changed as our knowledge develops about how planets are formed, how they develop, and how fast we’ll be able to travel through space.
When I ﬁrst began to read popularized books about astronomy, many of their authors seemed to think that planets were formed when two stars approached each other very closely, exerting such a strong gravitational attraction on each other that they pulled material out of each other, and that this hot, gaseous material then solidiﬁed and formed solid particles that orbited them. Since stars only rarely approach each other that closely, such events would be very rare, and therefore planetary systems should be very rare. This concept foundered when a Russian astronomer proved that the very hot, gaseous material, that would be pulled out of stars in such a manner, would not solidify, but would just dissipate in space.
It is now considered by astronomers that, as stars are formed by condensation out of interstellar matter by its mutual gravitational attraction, some particles rotate so fast around this proto-star that they do not become part of the star, but condense into much smaller, solid bodies. So much matter becomes part of the star (or, in many cases, stars) that pressures and temperatures at its core increase to the point where light atoms fuse into heavier atoms, thus making the star shine. But the smaller bodies in orbit around the proto-star do not get that hot, and instead solidify into planets.
Astronomers were then of the opinion that many planetary systems could exist, but that they could not be seen from Earth because the stars they orbited were so bright that planets could not be seen in their glare. Then evidence of planets by other means were sought. Gravitational evidence of planets could be determined by their minute inﬂuences on the motions of the much larger stars which they orbit. A slight but regular-dimming of a star indicated that it was caused by a planet orbiting around that star and slightly dimming its light each time it passed in front of the star. By such means, estimations could be made of the planet’s period, its size, and its distance from its star. This has led to the compilation of the Exoplanet Catalog, which now lists over 3,400 planets orbiting stars other than our Sun.
The star nearest to our Sun is actually a triple star system, Alpha Centauri, at a distance of 4.24 light years. (This is the distance light would travel in 4.24 years. One light year is about ten trillion kilometers, or six trillion miles.) The brightest of these three stars, Alpha Centauri A, is like our Sun a yellow star, somewhat larger and brighter than the Sun. Alpha Centauri B is a little smaller and fainter than the Sun. Alpha Centauri C is a much smaller and fainter red dwarf star, usually called “Proxima Centauri” or just “Proxima”, Latin for “nearest”, since it is by a small amount slightly nearer to the Sun than the other two Alpha Centauri stars. The Alpha Centauri system, which appears to the unaided eye as a single star, is the third brightest star in Earth’s night sky. It is too far south to be seen from north of about 30° N latitude. Medieval Arabian astronomers named it Rigel Kentaurus, “Foot of the Centaur”, for its position in that constellation.
Proxima is known to have at least one planet, about the size of our Earth. It would have to be quite near to Proxima, a cool red dwarf star, in order to have a climate warm enough to support life, since liquid water would be necessary for life to appear there.
There has recently been discovered another star, which has at least seven planets. It is also a red dwarf, the most common sort of star in the universe. That type of star is also the easiest to search for planets, since it is small and thus more likely to be affected by their much smaller gravitational ﬁelds. This star had been given the cumbersome and uninformative name “Trappist-1”, for the Chilean telescope used in its discovery.
The fact that a Chilean telescope was used indicates that Trappist-1, like Alpha Centauri, is too far south in the sky for an American telescope to be useful. And Trappist-l is about 40 light years away, which means that even if one of its planets is home to a technological civilization, messages to it would require 40 years to get there, and another 40 years for a reply. And we have no reason at all to believe that the velocity of light can be exceeded by any means whatsoever.
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this page last updated 6 May 2017
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