In our solar system, we have two kinds of worlds. Small, warm, rocky worlds populate the inner area, while the outer area has cold gas giants. Intuitively this makes a lot of sense. When the planetary system was forming, the Suns light and heat should have pushed much of the gas toward the external system, leaving much heavier dust and rock to form the inner worlds. Giants could just grow in the cold, dark external planetary system. However we now understand our planetary system is more the exception than the rule. Lots of galaxy have large gas worlds that orbit close to their stars. These hot Jupiters and hot Neptunes are unlike anything in our solar system, and astronomers are keen to understand what they might be like.
One method to study warm gas worlds is by taking a look at the distribution of known exoplanets. Currently, close orbiting worlds are easier to discover than more far-off ones, so we have lots of examples of gas worlds that closely orbit their stars. As a recent research study shows, theres a fascinating pattern when it comes to Neptune-sized worlds.
Few Neptune-sized worlds are discovered near A-type stars. Credit: Steven Giacalone, utilizing NASA data
Close orbiting warm Neptunes can be discovered around a number of stars, consisting of A-type stars, which release powerful ultraviolet and x-ray light. These worlds do not have orbits much shorter than about 3 days. On the other hand, we know of several hot Jupiters with extremely short orbital periods. Why is that?
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In our solar system, we have two types of planets. When the solar system was forming, the Suns light and heat need to have pushed much of the gas towards the outer system, leaving heavier dust and rock to form the inner worlds. Giants might just grow in the cold, dark outer solar system. Numerous star systems have large gas planets that orbit close to their stars. These hot Jupiters and hot Neptunes are unlike anything in our solar system, and astronomers are eager to understand what they may be like.
To respond to that concern, the group looked at how ultraviolet light can connect with the thick environment of a gas world. A-type stars have relatively short life times of only about a billion years. From their design, the team discovered that warm gas worlds more massive than about 8 Earths can keep their atmosphere for the lifetime of an A-type star, which is why we observe both warm Neptunes and warm Jupiters. The team also discovered that “hot” worlds with orbital periods of less than a couple of days would be removed of their atmosphere if they are less huge than about 14 Earths. So, while bigger Jupiter-like worlds have enough gravity to hold on to their environments, smaller sized Neptune-like worlds dont. This would explain the “hot Neptune desert,” and why we do not see numerous close-orbiting Neptunes around hot stars.
There ought to be plenty of rocky worlds orbiting A-type stars if the groups design is right. These cores of removed Neptunes cant be detected by present telescopes however could be seen by future generations of telescopes. The group would likewise like to find more hot Neptunes, so they can much better comprehend whether these worlds form close to their star, or whether their orbits shift inward in time.
Referral: Giacalone, Stephen, et al. “HD 56414 b: A Warm Neptune Transiting an A-type Star.” The Astrophysical Journal Letters 935.1 (2022 ): L10.
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