Astronomy/Orange dwarf stars etc.
It has been mentioned many times that the most likely solar systems with life on some of the planets within are ones with orange dwarf stars, because such stars have the longest longevity of most stars, other than red dwarf stars. Now, so far, all the reported so-called "habitable" planets in other solar systems have been cited as circling red dwarf stars.But this means having to have one face of the planet permanently facing the red dwarf star and being rather too close for comfort in order to be in a habitable zone etc. Why is it so much more difficult to find habitable planets surrounding non-red-dwarf-stars?
Lastly, I used to read James White's marvellous Galactic Space Hospital series, all about numerous sentient alien species living in chlorine/hydrogen/oxygen/H20 etc. atmospheres, or in gravities of less than 1g or far more than 1g, and so on. Are current scientists right in thinking that all alien life will have to be within Earth-norms of some sort as in a form of parallel evolution? Or was James White correct in thinking that alien, extraterrestrial life would truly be alien by nature?
I am not sure you're aware, but the number of "habitable" planets found thus far is exceedingly small. Based on recent papers delivered at the 225th meeting of the American Astronomical Society (AAS) in Seattle, two new planets (deemed "Goldilocks" candidates) have been found. These are so defined because they meet the same conditions for life support as here on Earth (which lies precisely within a habitable zone at the right distance from our star, the Sun).
The two new habitable planet candidates reported at the AAS meeting are:
1) Kepler - 438b - which is only 12 percent larger than the Earth in diameter or
(0.12) (8,000 mi.) = 960 miles greater
This means a larger density than would be expected from a gas planet, hence likely a rocky planet (astro-geologists assign about a 70 percent chance of being rocky).
2) Kepler - 442 b is a bit larger - about 33 percent enhanced over Earth's diameter or:
(8,000 mi.) / 3 = 2667 miles greater or 10, 667 mi.
This means a larger surface gravity (g) and at least a 60 percent chance of being rocky. Interestingly also, both planets orbit a red dwarf star (late K or M spectral type) - not a G2 spectral type like our Sun. The red dwarf stars being at much lower surface temperature (4,000 F vs. 11,000 F for Sun) would require any planets to be much closer to arrive within the habitable zone and this is what has been found.
As noted at the AAS meeting where 554 new planetary candidates were announced, the grand total now comes to 4, 175.
However, of those thousands only eight are within the "Goldilocks" (habitable) zone of their host stars, including the two new ones identified above.
I am not sure where you got your information on orange and red dwarfs but let me clarify that red dwarfs (late K, M types) can be as long -lived as orange (G0-G4) types. So can support the long time needed for evolution. It all depends on where the planet is in relation to the star and how stable the orbit is within the habitable zone. Also, I don't know where you got the information that a planet must be in some synchronous orbit about the host star. In fact, we have no information as yet on any of the orbital revolution periods so cannot assume such rotation. SO one face does not have to "permanently" face the red dwarf star while the other half is away from it. (Which would be a catastrophic condition for life to evolve).
As to your question 'why it's so hard to find planets around non-dwarf stars?' (red or orange) the answer is because higher spectral types mean much shorter lifetimes on the main sequence so less time to evolve life. Also, for whatever reason, the masses of the planets found so far on those stars seem to be far too large to support life (too high a g-value would make it difficult to evolve life like ours) or else are gas giants like Jupiter.
Regarding James White's alien life conjectures, while it's certainly possible (I never like to say 'impossible' ) that such exotic life might exist, most exo-biologists adhere to a conservative or orthodox view that any actual extraterrestrial life is more probably carbon -based. Thus, it would more plausibly follow along evolutionary lines as life forms have on Earth - which means beginning on a planet within a habitable zone as a necessary condition.