Curious Kids is a series for children of all ages. If you have a question you’d like an expert to answer, send it to This email address is being protected from spambots. You need JavaScript enabled to view it..

What is a dwarf planet? – Myranda, age 8, Knoxville, Tennessee

The word “planet” came from the ancient Greek words that mean “wandering star.” That makes sense, because for thousands of years, people have watched planets change position in the night sky – unlike stars, which appear fixed and unmoving to the naked eye.

That’s how the ancients discovered five of the planets: Mercury, Venus, Mars, Jupiter and Saturn. Astronomers using telescopes found Uranus in 1781, Neptune in 1846, and Pluto in 1930.

Solar system leftovers

I’m a space scientist with a passion for astronomy and the exploration of the Solar System. I received my Ph.D. in physics in 1994, about the time astronomers began to find more and more objects beyond Neptune, in the Kuiper belt. That’s a place in space that holds the “leftovers” of the solar system – particularly small icy bodies.

Three of those icy bodies – Eris, Haumea and Makemake – were discovered in the early to mid 2000s. They seemed large enough to be planets; all of them are roughly the same size as Pluto.

Astronomers then surmised that there were likely many more of these icy bodies in the Kuiper belt. They began to wonder: How many planets might we end up identifying in our solar system? Twenty? Thirty? A hundred? More?

Dwarf planet defined

In 2006, and after much debate, the International Astronomical Union came up with a new definition for a planet. And for the first time, the term “dwarf planet” was used.

Here’s what the IAU said: A planet has to orbit the Sun directly. It also must be large enough to have a round, or spherical, physical shape.

And the planet must “clear its neighborhood.” That means, aside from any moons it might have, the planet can’t share its orbit with other objects of comparable size.

An object that satisfies only the first two criteria – but not the last – is now called a dwarf planet.

Pluto is demoted

That’s why Pluto lost its status as a planet and is now classified as a dwarf planet. It failed the final item on the checklist – other icy Kuiper belt bodies are within its orbital path. The decision, a controversial one to be sure, is debated by scientists to this very day.

At the same time Pluto got demoted, another solar system object was promoted. Ceres, once considered an asteroid, is now classified as a dwarf planet. It’s nowhere near the Kuiper belt; instead, Ceres is in the main asteroid belt, orbiting between Mars and Jupiter.

Add them up – Pluto, Ceres, Eris, Haumea and Makemake – and that brings the number of dwarf planets in our solar system to five. But that list is sure to grow. Already, hundreds of candidates, nearly all in the Kuiper belt, potentially satisfy the criteria to be a dwarf planet.

About the dwarf planets

Dwarf planets are nothing like Earth.

As their name implies, they are much smaller. Pluto and Eris, the largest of the dwarfs, have less than one-fifth the diameter of the Earth.

They have less mass, too. For example, Earth has about 6,400 times more mass than Ceres. That’s like comparing two killer whales to a guinea pig.

And dwarf planets are cold. Pluto’s average temperature is around minus 400 degrees Fahrenheit (minus 240 Celsius).

Could life exist on a dwarf planet?

Three things are needed for life: liquid water, an energy source and organic molecules – that is, molecules containing carbon.

More than 100 miles (161 kilometers) below Pluto’s surface, an enormous ocean of liquid water may exist; this might also be true for other Kuiper belt worlds. Ceres also has subsurface water, remnants of what might have been an ancient global ocean.

Organic molecules, in abundance everywhere in our solar system, have been found on Ceres and Pluto.

But the one missing ingredient for all the dwarf planets is a source of energy.

Sunlight won’t work, particularly for the Kuiper belt dwarfs; they are simply too far away from the Sun. To reach the belt, the light must travel more than 2.7 billion miles (4.4 billion km). By the time the sunshine arrives at these distant worlds, it’s too weak to significantly heat any of them.

And all the dwarf planets are too small to hold the inner heat that remains from the solar system’s formation.

Yet scientists have discovered life on Earth in the most hostile places imaginable – near the bottom of the ocean, miles deep in the soil and even inside an active volcano. When it comes to life in our solar system, never say never.

Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to This email address is being protected from spambots. You need JavaScript enabled to view it.. Please tell us your name, age and the city where you live.

And since curiosity has no age limit – adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.

Vahe Peroomian, Professor of Physics and Astronomy, USC Dornsife College of Letters, Arts and Sciences

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Around the world, revolutionary changes are under way in transportation. More electric vehicles are on the road, people are taking advantage of sharing mobility services such as Uber and Lyft, and the rise in telework during the COVID-19 pandemic has shifted the way people think about commuting.

Transportation is a growing source of the global greenhouse gas emissions that are driving climate change, accounting for 23% of energy-related carbon dioxide emissions worldwide in 2019 and 29% of all greenhouse gas emissions in the U.S.

The systemic changes under way in the transportation sector could begin lowering that emissions footprint. But will they reduce emissions enough?

In a new report from the Intergovernmental Panel on Climate Change released April 4, 2022, scientists examined the latest research on efforts to mitigate climate change. The report concludes that falling costs for renewable energy and electric vehicle batteries, in addition to policy changes, have slowed the growth of climate change in the past decade, but that deep, immediate cuts are necessary. Emissions will have to peak by 2025 to keep global warming under 1.5 degrees Celsius (2.7 F), a Paris Agreement goal, the report says.

The transportation chapter, which I contributed to, homed in on transportation transformations – some just starting and others expanding – that in the most aggressive scenarios could reduce global greenhouse gas emissions from transportation by 80% to 90% of current levels by 2050. That sort of drastic reduction would require a major, rapid rethinking of how people get around globally.

The future of EVs

All-electric vehicles have grown dramatically since the Tesla Roadster and Nissan Leaf arrived on the market a little over a decade ago, following the popularity of hybrids.

In 2021 alone, the sales of electric passenger vehicles, including plug-in hybrids, doubled worldwide to 6.6 million, about 9% of all car sales that year.

Strong regulatory policies have encouraged the production of electric vehicles, including California’s Zero Emission Vehicle regulation, which requires automakers to produce a certain number of zero-emission vehicles based on their total vehicles sold in California; the European Union’s CO2 emissions standards for new vehicles; and China’s New Energy Vehicle policy, all of which have helped push EV adoption to where we are today.

Beyond passenger vehicles, many micro-mobility options – such as autorickshaws, scooters and bikes – as well as buses, have been electrified. As the cost of lithium-ion batteries decreases, these transportation options will become increasingly affordable and further boost sales of battery-powered vehicles that traditionally have run on fossil fuels.

An important aspect to remember about electrifying the transportation system is that its ability to cut greenhouse gas emissions ultimately depends on how clean the electricity grid is. China, for example, is aiming for 20% of its vehicles to be electric by 2025, but its electric grid is still heavily reliant on coal.

With the global trends toward more renewable generation, these vehicles will be connected with fewer carbon emissions over time. There are also many developing and potentially promising co-benefits of electromobility when coupled with the power system. The batteries within electric vehicles have the potential to act as storage devices for the grid, which can assist in stabilizing the intermittency of renewable resources in the power sector, among many other benefits.

Other areas of transportation are more challenging to electrify. Larger and heavier vehicles generally aren’t as conducive to electrification because the size and weight of the batteries needed rapidly becomes untenable.

For some heavy-duty trucks, ships and airplanes, alternative fuels such as hydrogen, advanced biofuels and synthetic fuels are being explored as replacements for fossil fuels. Most aren’t economically feasible yet, and substantial advances in the technology are still needed to ensure they are either low- or zero-carbon.

Other ways to cut emissions from transportation

While new fuel and vehicle technologies are often highlighted as decarbonization solutions, behavioral and other systemic changes will also be needed to meet to cut greenhouse gas emissions dramatically from this sector. We are already in the midst of these changes.

Telecommuting: During the COVID-19 pandemic, the explosion of teleworking and video conferencing reduced travel, and, with it, emissions associated with commuting. While some of that will rebound, telework is likely to continue for many sectors of the economy.

Shared mobility: Some shared mobility options, like bike and scooter sharing programs, can get more people out of vehicles entirely.

Car-sharing and on-demand services such as Uber and Lyft also have the potential to reduce emissions if they use high-efficiency or zero-emission vehicles, or if their services lean more toward car pooling, with each driver picking up multiple passengers. Unfortunately, there is substantial uncertainty about the impact of these services. They might also increase vehicle use and, with it, greenhouse gas emissions.

New policies such as the California Clean Miles Standard are helping to push companies like Uber and Lyft to use cleaner vehicles and increase their passenger loads, though it remains to be seen whether other regions will adopt similar policies.

Public transit-friendly cities: Another systemic change involves urban planning and design. Transportation in urban areas is responsible for approximately 8% of global carbon dioxide emissions.

Efficient city planning and land use can reduce travel demand and shift transportation modes, from cars to public transit, through strategies that avoid urban sprawl and disincentivize personal cars. These improvements not only decrease greenhouse gas emissions, but can decrease congestion, air pollution and noise, while improving the safety of transportation systems.

How do these advances translate to lower emissions?

Much of the uncertainty in how much technological change and other systemic shifts in transportation affects global warming is related to the speed of transition.

The new IPCC report includes several potential scenarios for how much improvements in transportation will be able to cut emissions. On average, the scenarios indicate that the carbon intensity of the transportation sector would need to decrease by about 50% by 2050 and as much as 91% by 2100 when combined with a cleaner electricity grid to stay within the 1.5-degree Celsius (2.7 Fahrenheit) target for global warming.

These decreases would require a complete reversal of current trends of increasing emissions in the transportation sector, but the recent advances in transportation provide many opportunities to meet this challenge.

Alan Jenn, Assistant Professional Researcher in Transportation, University of California, Davis

This article is republished from The Conversation under a Creative Commons license. Read the original article.