How do Scientists Measure the Temperature of Planets?

temperature of planets
Photo Credit: Matteo Fusco

When determining what the coldest planet is, we must first ask how the temperature of planets are measured. Knowledgable people immediately answer “Uranus of course; the temperature on Uranus is minus 371 degrees Fahrenheit”. How does anyone know that? Did we shoot thermometers at the planet or something?

Actually, scientists HAVE placed thermometers on some planets, most notably Mars. But for the more distant planets like Uranus, scientists estimate the temperature by looking at the light emitted by the planet. When estimating temperature, scientists don’t look at visible light; they look at infrared light, the light that has a longer wavelength than the red light that we can see.

red hot coals
Photo Credit: Michal Matlon

Planck’s law is a formula that calculates how much light is emitted at each wavelength from an object based on its temperature. This law is firmly founded on the basics of quantum mechanics, thermodynamics, and relativity. You can obtain an intuitive understanding of what the scientists are doing by looking at the coals of a fire. The hotter coals emit a lot of white light, less-hot coals glow red, and cool coals do not emit any visible light.

What Tools Are used to Calculate the Temperature of Planets?

Scientists use a device called a bolometer to convert the infrared light the planet emits at different wavelengths into distinct electrical signals, which can be converted into an estimate of the temperature of the planet. Similar technology is currently in use in thermal imagers and cameras, which are inexpensive and widely available. These devices are used for all sorts of purposes, such as checking for insulation leaks, determining if an infant has a fever, and observing wildlife in the dark.

Photo Credit: ActionVance

It’s rather astonishing that you can buy a thermal imager at Walmart for under $100 that works on the same principles that scientists use to estimate the temperatures of objects that are billions of miles away.

How Old is The Sun?

sun and clouds
Photo Credit: Pixabay

The Sun has been providing the Earth with sunlight for several billion years. Have you ever stopped and actually asked yourself How Old is the Sun? At some point, both celestial bodies will die, but while the Earth is expected to become uninhabitable within the next billion years, the Sun has a much longer lifespan.

The Sun is estimated to be 4.6 billion years old, making it one of the oldest entities in the Solar System. Despite the star’s age seeming like a massive number, the Sun is at the middle point of its life. Here’s a quick look at the Sun’s life cycle, from its formation to its ultimate demise.

What Is the Life Cycle of the Sun?

The Sun’s lifespan can be narrowed down to three phases: its formation, current state, and death.

The Formation

Simply put, the Sun was initially part of a hydrogen and helium molecular cloud before a nearby supernova’s shockwave forced it to become a star. In reality, that shockwave caused the cloud’s molecules to compress. Along with a rotating motion, a part of the cloud built up much heat and pressure in its core, leading to the formation of a star with disks surrounding it. 

That’s how the Sun (star) and the planets (disks) of the Solar System were formed.

NASA sun photo
Photo Credit: NASA

The Main Sequence

The Sun is almost at the midpoint of its main sequence phase, during which nuclear fusion occurs, turning hydrogen into helium. As the millennia pass, the star will continue depleting its core’s hydrogen while it also grows in size and emits an increasing amount of solar radiation.

The Sun’s main sequence phase is estimated to last 9-10 billion years. Considering it’s already 4.6 billion years old, it should have enough hydrogen in its core for almost 5 more billion years.

After Hydrogen Exhaustion

Once all the core’s hydrogen is exhausted, the Sun will spend a billion years expanding until it becomes a Red Giant. At that point, about 6 billion years away, the Sun’s surface will be close to Mars, and its luminosity will be 1000 times stronger. 

The Sun will spend around a billion years as a Red Giant before depleting its core’s helium, leading to an unstable phase and the beginning of its death.

How Old is The Sun and When Will The Sun Die?

The Sun’s death begins with an explosion (planetary nebula), followed by the star becoming a White Dwarf. From then on, estimates have shown that the Sun’s White Dwarf phase will last for billions, if not trillions of years, before turning into a Black Dwarf, at which point it won’t emit any light or heat.

How old is the sun
Rendering Credit: Pixabay


Even though the Sun is 4.6 billion years old, its core contains enough hydrogen to keep the star stable for another 5-6 billion years. Once the hydrogen is depleted, the Sun will gradually get larger before becoming a Red Giant. 

A billion years after the beginning of the Red Giant phase, the Sun will unstably increase in size before exploding into a planetary nebula and becoming a White Dwarf. At that stage, the Sun will be dead. However, it will still emit light and heat for billions or trillions of years before it turns into a Black Dwarf.

If you want to get a better idea of how we know how old the Sun is and what will happen to the Solar System, feel free to go through our in-depth analysis of the Sun’s and Solar System’s lifetime.

How Do We Know How Old the Sun Is?

The Sun has been shining for several billion years, allowing the Earth to become the only planet in the Solar System that can host life. But, how do we know how old the sun is when we can’t even get to its surface? Finding the exact point in time when the Sun formed was initially impossible, but there are methods we can use to get an estimate.

As technology advanced, scientists found several ways to pinpoint the Sun’s age, even though these were indirect methods. They all offered the same results, thus giving an accurate answer on the Sun’s age. Here are the two methods used to determine how old the Sun is.

How do we know how old the sun is
Rendering Credit: Brano

The Methods Used to Find How Old the Sun Is

Luckily enough, the entire Solar System and the Sun were formed within a few million years. Thus, by figuring out how old the Solar System is, estimates showed that the Sun is between 4.5-4.6 billion years old. Radioactive dating and Solar System evolution models are used to date the Solar System’s (and subsequently the Sun’s) age.

Radioactive Dating

Simply put, radioactive dating is the process of measuring the core decay of astronomical objects, such as meteorites, as well as Moon and Earth rocks. Considering that the Solar System is as old as the Sun, the radioactive dating results would reveal the star’s age.

While the oldest Earth rocks found were 3.8 billion years old, Moon rocks from the Apollo missions were determined to be 4.5 billion years old. Additionally, when dating ancient meteorites, the results gave a definitive answer; the Sun and the Solar System are around 4.6 billion years old.

Solar Eclipse
Photo Credit: Jongsun Lee

Solar System Evolution Models

In an attempt to replicate the results found from radioactive dating, scientists created computer models that simulated the Solar System’s evolution. By utilizing some parameters from the Sun, the results regarding the star’s age were identical to those of radioactive dating.

To understand how these computer models work, examining the aforementioned parameters is necessary. As the Sun ages, it produces different luminosity levels, allowing for accurate aging when added to a simulation model. Thus, by creating the stellar evolution models and crosschecking the luminosity levels, the Sun’s age was again determined to be 4.6 billion years.

How Do We Know How Old the Sun Is? Conclusion

Photo Credit: Pixabay

Finding out the Sun’s age was, at first, challenging. However, with the help of new technological advancements and the idea that the Solar System and the Sun were of similar age, scientists utilized two methods to discover how old the Sun is: radioactive dating and stellar evolution models.

With radioactive dating, the decay found in core elements of meteorites, Earth rocks, and Moon rocks was used to estimate their age, which would be equivalent to the Sun’s age. In addition to this, computer models of the Solar System’s evolution were developed, which came to the same conclusion as radioactive aging by utilizing the Sun’s current luminosity levels.

If you want to learn more about how long the Sun will last and what will become of the Solar System, feel free to check out our thorough analysis of the Sun’s life cycle.