Protoplanet facts for kids

Vesta, a protoplanet that survived!

A protoplanet is like a baby planet. It's a large space object that forms inside a protoplanetary disk, which is a disk of gas and dust around a young star. These baby planets get hot enough inside to melt. This melting causes heavier materials to sink to the center and lighter materials to rise, making the protoplanet have different layers. Scientists believe protoplanets grow from smaller pieces of rock and dust called planetesimals that crash into each other and stick together. Eventually, these protoplanets can grow into full-sized planets.

How Planets Are Born: The Planetesimal Idea

A planetesimal is a small object made of dust, rock, and other materials. They can be from a few meters to hundreds of kilometers wide.

From Dust to Planetesimals

Scientists think that when a star is very young, it is surrounded by a flat disk of gas and dust. This is called a protoplanetary disk. Over time, the force of gravity pulls these tiny bits of material together. They slowly form bigger and bigger clumps. Eventually, some of these clumps become large enough to be called planetesimals.

Growing into Protoplanets

It is believed that these planetesimals then started to crash into each other. These collisions created a few hundred larger objects called planetary embryos. These embryos were like early versions of planets. Over millions of years, these planetary embryos kept colliding.

Scientists think that the first collisions made a new group of embryos. These new ones were fewer in number but much larger. This process continued, with each new group having fewer but even bigger embryos. Finally, only a few very large embryos were left. These last few collisions helped to finish building the full-sized planets we see today.

Making Layers: Planetary Differentiation

Early protoplanets had more radioactive elements inside them. These elements slowly break down over time, which creates heat. This heat, along with impacts from other objects and the pressure of gravity, caused parts of the protoplanets to melt as they grew.

When a protoplanet melted, the heavier materials, like iron, sank to its center. Lighter materials, like rocky silicates, floated up to the surface. This process of separating materials into layers is called planetary differentiation. We can see evidence of this in some meteorites. Their makeup shows that some asteroids also went through this layering process.

Protoplanets in Our Solar System

In our own Solar System, scientists believe that hundreds of planetary embryos were formed from planetesimal collisions. These embryos were similar in size to Ceres or Pluto. They weighed about 10²² to 10²³ kilograms and were a few thousand kilometers across.

How Our Moon Formed

One big idea about how our Moon formed is called the giant impact hypothesis. This idea suggests that a huge protoplanet, sometimes called Theia, crashed into Earth very early in the Solar System's history. This massive collision is thought to have created the Moon.

Surviving Protoplanets Today

In the inner part of our Solar System, three protoplanets have mostly survived until today. They are the asteroids Ceres, Pallas, and Vesta. Another asteroid, Psyche, might be what's left of a protoplanet that lost its outer rocky layers in a violent crash. The asteroid Metis might have a similar story. Also, Lutetia has features that look like a protoplanet.

Some Kuiper-belt dwarf planets are also sometimes called protoplanets. We have found iron meteorites on Earth. This suggests that there were once other protoplanets with metal cores in the asteroid belt. These protoplanets likely broke apart, and their pieces became the source of these meteorites.

Finding Protoplanets in Space

In 2013, astronomers saw the first possible protoplanet forming around a distant star called HD 100546. Later observations suggest there might be several protoplanets in that star's gas disk.

Another possible protoplanet, AB Aur b, might be a gas giant just starting to form. It orbits the star AB Aurigae. AB Aur b is one of the largest exoplanets found. It has a very wide orbit, much farther from its star than Neptune is from our Sun. Studying AB Aur b could change how we think planets are made. It was seen using the Subaru Telescope and the Hubble Space Telescope.

Clues in Protoplanetary Disks

Scientists look for clues in protoplanetary disks that might show a protoplanet is there. These clues include rings, gaps, spirals, clumps of dust, and shadows. These features are not fully understood, so they don't always prove a protoplanet is present.

A new way to study how protoplanets affect their disks is by looking at gas velocity maps. These maps show how gas is moving in the disk. HD 97048 b was the first protoplanet found this way. It caused a "kink" in the gas velocity map. Other disks, like those around IM Lupi or HD 163296, show similar kinks. Another possible exoplanet, HD 169142 b, was first seen in 2014. It shows several signs that it is a protoplanet.

Confirmed Protoplanets (Examples)

Here is a list of some confirmed protoplanets that scientists have identified:

See also

In Spanish: Protoplaneta para niños

• Accretion (astrophysics)
• Fusor (astronomy)
• Mesoplanet
• Planetesimal