Missing Worlds: Hot and Cold

In many cases, new exoplanets are identified according to their temperature or size, and what they are most familiar to; consequently, there are plenty of hot Jupiters, super-Earths, and hot Neptunes out there in the universe.

The nomenclature for this type of planet is very imprecise; you can often discover a new planet class in Google simply by combining the name of an existing planet with a select list of adjectives.

So, for example; in addition to the above three examples, a hot Uranus (GJ3470b, discovered last year), and a colossal super Jupiter, currently being consumed by a black hole, are both known to astronomers.


>> above: Earth and a super-Earth, 55 Cancri e. Image by NASA.

In keeping with the theme started by the previous post, here are a number of planet types curiously absent from our own Solar System.

  • Temperature Swings

hot Neptune describes a modest gas giant in close orbit around its star – hence the ‘hot’ portion of its name. The archetype of this class is Mu Arae c. However, there is evidence to suggest that the exoplanet, Gliese 581 b, is a hot Neptune.

One element of the hot Neptune is almost paradoxical; these planets retain water ice in roasting temperatures due to the extreme pressures at depth.

Hot Jupiters are rather more enigmatic. Astronomers believe that these strange planets form in the outer regions of their solar systems and migrate inwards, as there is not enough material to form a gas giant in close proximity to a star; it would inevitably be consumed by the sun’s heat and gravity.


>> above: 51 Pegasi b, the archetype of the hot Jupiter. Image from Celestia.

Giant, hellish worlds, hot Jupiters are typically located close enough to their parent star to complete an orbit in just a few days.

This planet type continues to defy convention. “The hot Jupiters are beasts to handle”, explained Nicole Lewis, of the Massachusetts Institute of Technology (MIT), to Space.com. “We are just starting to put together the puzzle pieces of what’s happening with these planets.”

One of the more unusual hot Jupiters, HAT-P-2b, has an eccentric orbit, meaning that at its closest point, the planet is just 3 million miles from its star; at its most distant, it can be more than 9 million miles away.

Consequently, the planet endures frequent, violent temperature swings, depending on the time of its year.

  • Giant Cannonballs

Moving on, a super-Earth is a rather more ambiguous term for a planet larger than our own, but smaller than the Sun’s second tier gas balls, Uranus and Neptune.

The moniker is inherently disappointing; it does not imply the presence of life or any physical resemblance to our own planet, other than its approximate size.


>>above: the distant (1,200ly) super-Earth, Kepler-62e, believed to be located within the habitable zone of its star. Image by NASA.

The smallest super-Earths are believed to be rocky bodies with far more aggressive geological processes than their namesake. The larger examples may not resemble Earth at all, becoming small gas giants, a body that has been alternatively coined mini-Neptune or gas-dwarf.

Particular super-Earths, such as Gliese 581 c, could also resemble giant cannonballs, according to Sara Seager, of the MIT, or be made entirely of carbon.

>>this is part II of Missing Worlds; start at the beginning.


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