Yargonian

The Yargonians are a near-human species from the planet Yargo, ruled by their eponymous supreme leader Yargo. They are characterized as peaceful, yet extremely arrogant and convicted of their own superiority, both physical and spiritual. Like the Vulcans, they have dedicated their cultural development to the elimination of all emotions and pursuit of spiritual and intellectual growth. A combination of eugenics and advanced medical knowledge has made them physically healthful, naturally living a lifespan of 300 years with no physical senescence. They also lack an appendix and are said to be incapable of crying, although it has been shown that in circumstances of extreme trauma, tears may still be shed. All Yargonians have vivid emerald-green eyes and dress in identical (albeit colorful) clothes to avoid shallow displays of individuality, and all the males shave their heads for similar reasons.

The only aspect in the life of a Yargonian which can be described in emotional terms is their extreme, absolute and disproportional devotion to their leader Yargo, whom they speak of and treat as a literal god, given his supposed intellectual superiority to all other individuals of any race or planet. The superiority of the Yargo is never challenged, and there are strict rules against touching him, speaking to him unless he speaks first, or displeasing him in any fashion. A typical Yargonian once admitted that, if she were to live the rest of her life away from Yargo, life itself would lose meaning; illustrating the extent to which their devotion goes. It has been hinted that the Yargo possesses hypnotic suggestion powers, as his presence has some disruptive effect on the mind of Humans and Yargonians alike, causing all present to feel a deep and instantaneous admiration for every aspect of his being. Physically, he is distinguished from the others of his race by his eyes, which are blue rather than green, and it is hinted that they might be the source of his hypnotism. It's also likely that he has telepathic abilities, as it has been stated that he can communicate with all life, including non-sapient beings.

The Yargonians travel through space at FTL speed in Flying Saucers. Before the discovery of the Venusians, they believed themselves to be the only species to have developed space vessels, although they did keep diplomatic contact with non-spacefaring races, including the Martians. They never made contact with the Earth population, though, believing them to be so primitive they are only worth for distant studying.

Despite having access to advanced technology, including the ability to instantaneously communicate across interstellar distances; the Yargonian vessels are rather lacking in sensors and other detection devices, as shown by them being unable to detect the approach of other ships by any means other than looking at the windows; and the fact that the dense Venusian atmosphere is enough an obstacle to completely prevent their exploration of that planet. Since they claimed to be the only spacefaring power in their area of space including the Sol System, this apparent lack of interest in sensor technology might be justified. Still, they did take caution to travel always in scouts, showing that they at least considered the possibility of unknown enemies that might have to be dealt with.

Behind the scenes
At the end of the book, it is very strongly implied that the entire science fiction plot may have been a fantasy created by the protagonist's mind; meaning that all alien species described never existed even in-universe. This likely explains the inaccurate statements about astronomy made by the supposedly advanced Yargonians.

In the story, they identify the Earth's sun as a cepheid, specifically of the "orange dwarf" kind, and state that these are the stars most likely to go nova. They speak of novae as if they were an unnatural and hard-to predict phenomenon and believe the sun spots and solar flares are somehow linked to atomic tests realized on Earth, and that both phenomenon are indications that the sun is about to explode in the next few decades. In reality, the sun is a relatively stable yellow dwarf, all of which naturally expand at the end of their lives (the phenomenon the Yargonians incorrectly identify as a "nova"), which in the sun's case is not predicted to happen in the next five billion years and is not related to sun spots.

If the experiences lived by Janet Cooper are to be interpreted as real within the book's universe, the only possible explanations for these inaccuracies are that either the Martians and Yargonians were lying for some reason, or both had extremely inaccurate ideas about astronomy, which in the Yargonians' case at least might be explainable by their ridiculously extreme faith in Yargo's wisdom which might lead to them adopting any scientific concept proposed by him no matter how wrong. It is easy to see how the absurdity of challenging Yargo or even disagreeing with him would quickly result in the stagnation of Yargonian science, contrary to their own claim of being a race dedicated to intellectual growth and the pursuit of knowledge.