Gas giant | Types of planets – Exoplanet exploration: planets beyond our solar system

What is a gas giant?

The basics

What is a gas giant?

A gas giant is a large planet composed mainly of helium and/or hydrogen. These planets, like Jupiter and Saturn in our solar system, do not have hard surfaces and instead have gases swirling above a solid core. Gas giant exoplanets can be much larger than Jupiter and much closer to their stars than anything found in our solar system.

For most of human history, our understanding of planetary formation and evolution was based on the eight (or nine) planets of our solar system. But over the past 25 years, the discovery of more than 4,000 exoplanets, or planets outside our solar system, has changed everything.

Gas giants, such as Jupiter or Saturn in our solar system, are composed mainly of helium and/or hydrogen. Gas giants closer to their stars are often called “hot Jupiters”. More variety hides in these broad categories. Hot Jupiters, for example, were among the first types of exoplanets discovered — gas giants like Jupiter, yes, but orbiting so close to their stars that their temperatures reach thousands of degrees (Fahrenheit or Celsius). These large planets orbit so tightly that they cause a pronounced “wobble” in their stars, pulling their stellar hosts one way or another, and causing a measurable change in the spectrum of starlight. This made hot Jupiters easier to detect in the early days of planet hunting using the radial velocity method.

Get to know some gas giants

Get to know some gas giants

The heat of KELT-9b is too high even for the molecules to remain intact.


Kepler-7b has about the same density as polystyrene.


Similar in size to Jupiter, these gas-dominated planets orbit extremely close to their parent stars, circling them in as little as 18 hours. We don’t have anything like it in our own solar system, where the planets closest to the Sun are rocky and orbit much farther out. The questions about hot Jupiters are as vast as the planets themselves: do they form near their stars or farther away before migrating inwards? And if these giants migrated, what would that reveal about the history of the planets in our own solar system?

To answer these questions, scientists will need to observe many of these hot giants very early in their formation. The detection of exoplanet HIP 67522 b, considered the youngest hot Jupiter ever discovered (in June 2020), could expand our understanding. It orbits a well-studied star that is about 17 million years old, which means the hot Jupiter is probably only a few million years younger, while most known hot Jupiters are older. a billion years. The planet takes about seven days to orbit its star, which has a mass similar to that of the Sun. Located just 490 light-years from Earth, HIP 67522 b is about 10 times the diameter of Earth, close to that of Jupiter. Its size strongly indicates that it is a gas-dominated planet.

This discovery offers hope to find more young, hot Jupiters and to learn more about the formation of planets in the universe.


Migrant giants?

Migrant giants?

There are three main hypotheses about how hot Jupiters come so close to their parent stars. The first is that they simply form there and stay put. But it’s hard to imagine planets forming in such an intense environment. Not only would the scorching heat vaporize most material, but young stars would frequently erupt with massive explosions and stellar winds, potentially scattering emerging planets.

Gas giants are more likely to grow farther from their parent star, beyond a boundary called the snow line, where it is cold enough for ice and other solid materials to form. Jupiter-like planets are composed almost entirely of gas, but they contain solid cores. It would be easier for these nuclei to form beyond the snow line, where frozen material could clump together like a growing snowball.

The other two hypotheses assume that this is the case and that hot Jupiters then move closer to their stars. But what would be the cause and timing of the migration?

One idea posits that hot Jupiters begin their journey early in the planetary system’s history while the star is still surrounded by the disk of gas and dust from which it and the planet formed. In this scenario, the gravity of the disk interacting with the mass of the planet could disrupt the gas giant’s orbit and cause it to migrate inward.

The third hypothesis holds that hot Jupiters approach their star later, when the gravity of other planets around the star can drive the migration. The fact that HIP 67522 b is already so close to its star so soon after its formation indicates that this third hypothesis probably does not apply in this case. But a hot young Jupiter isn’t enough to settle the debate about how they all form.

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Evolve with the stars

Evolve with the stars

In 2007, astronomers using NASA’s Spitzer Space Telescope found evidence showing that gas giant planets form rapidly, within the first 10 million years of a Sun-like star’s life.

Gas giants could get their start in the disk of gas-rich debris that surrounds a young star. A nucleus produced by collisions between asteroids and comets provides a seed, and when that nucleus reaches sufficient mass, its gravitational pull quickly pulls gas from the disk to form the planet.

Scientists using Spitzer and ground-based telescopes searched for traces of gas around 15 different solar-type stars, most with ages ranging from 3 million to 30 million years. With the help of Spitzer’s infrared spectrometer instrument, they were able to search for relatively hot gases in the inner regions of these star systems, an area comparable to the area between Earth and Jupiter in our own solar system. They also used ground-based radio telescopes to search for cooler gases in the outer regions of these systems, an area comparable to the area around Saturn and beyond.

All of the stars in the study, including those only a few million years old, have less than 10% of Jupiter’s mass in gas swirling around them. This indicates that gas giant planets like Jupiter and Saturn have already formed in these young planetary systems, or they never will.

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