A bright dot in the evening sky might be a planet or a star. Appearance alone tells us little: even Jupiter, the Solar System’s largest planet, looks like a tiny light from far away. And, like the Sun, it consists mostly of hydrogen and helium. What is it missing that keeps it from becoming a second Sun?
The Sun sustains its glow through ongoing nuclear fusion of hydrogen deep inside it. Planets lack this energy source. In visible light, we see our planets mainly by reflected sunlight, but they also emit their own thermal radiation. So “shines by itself” is an incomplete explanation: we need to ask where the energy comes from. NASA: the lives of stars, NASA: Jupiter’s composition.
Seeing light does not yet identify its source
Imagine a lamp and a ball it illuminates. Light reaches your eye from both, but there is only one source of illumination. Similarly, sunlight reflects off planetary clouds and surfaces. This path is familiar from our explanation of sunlight and moonlight.
But the ball is not perfectly “non-emitting.” At ordinary temperatures, it loses energy mainly as infrared radiation: electromagnetic waves our eyes cannot see. Planets radiate heat too. Absorbed sunlight warms them, and some energy comes from their interiors. Reflection sends back part of the incoming light; thermal emission depends on the body’s own temperature. These are different processes. Catling and Kasting: planetary energy sources, §2.1.
An infrared camera can therefore detect a planet through its own radiation without that planet being a star. The JIRAM instrument aboard Juno, for example, measures heat escaping from Jupiter. Emission describes what a body gives off; its energy source explains how it can keep doing so over time. NASA: Jupiter in the infrared.
How gravity prepares a star’s energy source
To understand the Sun, imagine a large gas cloud contracting under its own gravity. Contraction heats the material: energy from its gravitational infall becomes internal energy. This is how a future star can initially shine before sustained hydrogen fusion begins.
Inside a massive gaseous body, inner layers support the weight of outer ones. Pressure and temperature increase inward. If conditions are sufficient, hydrogen nuclei can participate in a chain of reactions whose net result is helium formation and the release of energy. This is nuclear fusion: atomic nuclei change, unlike ordinary chemical burning with oxygen.
When hydrogen fusion powers a star over a long period, it is on the main sequence, the longest stage of a star’s life. That is the Sun’s current stage. Pressure in hot matter resists contraction, while energy produced inside replenishes losses through radiation. These are two related balances: supporting the layers’ weight and supplying energy. NASA: stellar birth and the main sequence.
The center of the Sun is about 15 million degrees Celsius. This describes our star; it is not a universal, exact temperature at which any sphere suddenly becomes a Sun. Conditions throughout the body and its ability to sustain reactions matter. NASA: the Sun’s structure.
Why Jupiter’s hydrogen is not enough
Jupiter is a useful test of this explanation. It is rich in the same main fuel as the Sun, but having hydrogen does not create the conditions for sustained hydrogen fusion. Jupiter’s mass is too low for its own gravity to establish the required core conditions. A “spark” above its clouds would not add mass. NASA: Jupiter’s formation and composition.
To convey the scale, the lower stellar mass limit is often rounded to about 80 Jupiter masses. This is a guide for hydrogen–helium bodies, not an exact switch for every composition and history. Jupiter is nowhere close to being a small star. Earth is much less massive still, and it is rocky. It will not become a second Sun simply by warming up: external heating creates neither the necessary mass nor a self-sustaining stellar energy source. JPL: planets, brown dwarfs and stars.
Branch · Comparing massesOptional · you can skip this
How far Earth is from stellar mass
Mass describes the amount of matter, not the diameter in a picture. JPL’s reference masses put one Jupiter at roughly 318 Earth masses. Let us write these mass units as M_(J) and M_(⊕), respectively. For our rounded reference value:
M_(J) ≈ 318 M_(⊕)
80 M_(J) ≈ 80 × 318 M_(⊕)
80 M_(J) ≈ 25 000 M_(⊕).
We multiply the number of Jupiters by the mass of one Jupiter expressed in Earth masses; the result retains the unit M_(⊕). We round because the starting “80” is approximate too. JPL: Earth’s and Jupiter’s masses.
This is a scale comparison, not a recipe: an “enlarged Earth” retaining its rocky composition is not a model of an ordinary hydrogen star. And even the smallest star is not a copy of the Sun. The calculation shows why a few extra degrees or a change in appearance cannot erase the difference.
Where does Jupiter’s internal heat come from?
Gravitational assembly and contraction heated the young planet. A large, hot world can release stored energy for a very long time; contraction and the redistribution of material can also supply heat. Sustained hydrogen fusion is not required. Comparing it with a hot object cooling slowly explains an energy reserve, but does not describe the full complexity of planetary interiors. Catling and Kasting, §2.1.1.
We can test this through energy accounting. Cassini observations from 2000–2001, analyzed by Li and colleagues in 2018, showed Jupiter emitting about twice as much thermal energy as it absorbed from the Sun. The comparison is specifically between thermal emission and absorbed solar energy, without adding reflected light. The excess reveals an internal contribution; it does not by itself prove nuclear fusion. Li et al.: Jupiter’s energy budget, Table 1.
Brown dwarfs lie between Jupiter and stars
A body can be much more massive than Jupiter and still not sustain ordinary hydrogen fusion. Such substellar objects are called brown dwarfs. They too radiate energy and cool over time. Diameter is a poor judge here: very different masses need not mean equally different sizes. Review of the working exoplanet definition, §2.4.
A brown dwarf may nevertheless fuse deuterium during part of its life. Deuterium is a rare form of hydrogen with an extra neutron in its nucleus. This reaction can occur under less demanding conditions than sustained conversion of ordinary hydrogen into helium. Deuterium supplies are limited: the dwarf may deplete them and continue cooling. Thus “once had nuclear reactions” and “is powered over a long period by ordinary hydrogen fusion” are different properties. Spiegel, Burrows and Milsom: deuterium fusion models.
Four bodies — different energy sources
Sun
- What we see / detect
- Its own visible and other radiation
- Where the energy comes from
- Hydrogen fusing into helium in the core
- Sustained fusion of ordinary hydrogen
- Yes
Earth
- What we see / detect
- Reflected sunlight + thermal IR
- Where the energy comes from
- Absorbed sunlight + internal heat
- Sustained fusion of ordinary hydrogen
- No
Jupiter
- What we see / detect
- Reflected sunlight + thermal IR
- Where the energy comes from
- Absorbed sunlight + cooling of a hot interior
- Sustained fusion of ordinary hydrogen
- No
Brown dwarf
- What we see / detect
- Its own radiation, mainly IR in cool objects
- Where the energy comes from
- Cooling; deuterium fusion may occur during part of its life
- Sustained fusion of ordinary hydrogen
- No
Branch · Classification boundariesOptional · you can skip this
Why the number 13 cannot define a planet by itself
About 13 Jupiter masses is often used as a boundary between planets and brown dwarfs based on the ability to undergo appreciable deuterium fusion. This is a different boundary from the roughly 80 masses for a star. Models show that the deuterium threshold depends on composition and on what fraction of deuterium counts as sufficient “burning.” It is not an unconditional rule that “12.9 means planet; 13.1 always means a different kind of world.” Spiegel et al., conclusions.
The name planet also needs a system context. The IAU’s 2006 definition for the Solar System requires an orbit around the Sun, a nearly round equilibrium shape, and gravitational dominance in the orbital neighborhood. Lack of fusion alone is insufficient: the Moon lacks it too, but is a satellite. JPL: the planet definition.
Exoplanets have a separate IAU commission working definition, explained in a 2022 review. It uses the deuterium limit, an orbit around a star, brown dwarf or stellar remnant, and further requirements, including a mass ratio below roughly 1/25 of the central body’s mass. Names for free-floating planetary-mass objects and the origins of borderline bodies remain topics of discussion. Here we are explaining the physics of the Sun and planets, rather than promising to sort the entire universe by one number. Lecavelier des Etangs and Lissauer, §§3–4.
What an observation lets us conclude
Suppose a telescope detects a body near a distant star that emits more heat than the starlight it absorbs. Have we already proved that it is another star? No. The excess points to internal energy, but Jupiter demonstrates that cooling of a hot interior can supply it. We need other data, including mass, spectrum and an age estimate, along with a physical model.
And if a planetary dot twinkles? That does not change its class either: twinkling depends on Earth’s atmosphere and the source’s apparent angular size. A convenient observational clue is not necessarily the cause of a physical distinction.
A planet can be gaseous, hot and emit its own thermal radiation. The Sun differs in its ability to sustain ordinary hydrogen fusion over a long period, thanks to conditions established by its mass. This comparison concerns a main-sequence star: stellar remnants, such as white dwarfs, can shine without this fusion and do not thereby become planets. NASA: later stellar evolution.
Now return to the map of the Solar System and try to explain why the Sun and Jupiter belong to different classes despite their similar main ingredients. The key is energy source and mass, rather than the color or size of a circle.