Cara Lucy O'Connor has a concern.

Pluto used to be one of the nine planets in the solar system, and it isn't anymore.

That's not right, said the little girl from Ireland. So Cara, with the help of her teacher, wrote a letter to NASA hoping to persuade the space agency to "make Pluto a planet again."

"I listened to a song and at the end of it the song said 'Bring Pluto Back' - and I would really like that to happen," Cara wrote.

She went on to explain that in 2006, Pluto was reclassified as a dwarf planet, "a type of planet that isn't big enough to clear its orbit".

She talked about the Kuiper belt, a doughnut-shaped ring beyond Neptune where many dwarf planets are located.

"I really think Pluto should be a main planet again like Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus & Neptune, because in one video I watched called 'Let's go meet the planets,' Pluto was at the very end," she wrote.

In another video, she said, Pluto "was put in the trash can and was scared by planet Earth."

"This was really mean," Cara wrote, "because no one or no planet or dwarf planets should be put in the trash can."

Cara dreams of becoming a NASA astronaut and "visit all the main planets including Pluto". But in the meantime, she told NASA, "you need to fix this problem for me."

Though not likely to result in a dramatic change in Pluto's demoted status as a dwarf planet, the 6-year-old's letter did not fall on deaf ears. James Green, director of NASA's Planetary Science Division, wrote her back.

"I agree with you that Pluto is really cool - in fact, who would have believed that Pluto has a heart? … It's a fascinating world that appears to be constantly changing. To me, it's not so much about whether Pluto is a dwarf planet or not; it's that Pluto is a fascinating place that we need to continue to study," Green wrote.

"I hope that you will discover a new planet," he added, "and I trust that if you continue to do well in school we will see you at NASA one of these days."

Cara, who hopes to discover a planet and name it Planet Unicorn, often peppers her teachers with questions about why black holes exist, or whether the moon landing really happened.

"She had the most interesting mind. She asked questions that I couldn't answer," said Sarah O'Donovan, who taught Cara last year at Glasheen Girls' School in Cork, Ireland.

"She's always interested in things that are far, far above her level."

Cara said she's been interested in space and astronomy since last year. She loves learning about "the forming of the planets," she said.

Her interest was piqued even more after listening to songs and watching videos about Pluto. So one day, she asked O'Donovan to help her write a letter to NASA.

Cara had not learned much yet about reading and writing, so her teacher wrote her words for her.

As Cara talked almost nonstop, O'Donovan typed, at times interrupting Cara so she could Google some of the bits of information the little girl was giving her to make sure they're accurate (They were, she said).

The letter was written and sent to NASA headquarters last April. O'Donovan said they also wrote to the European Space Agency after they did not receive a response from NASA. NASA wrote back months later.

The correspondence caught the attention of Irish media this week.

"I was happy," Cara, who wrote the letter when she was 5, told The Washington Post this week. "I was jumping up and down."

Green said he receives several letters every month, and he always answers them. Most recently, he said, he signed off on a letter to a young boy from Turkey who was quite concerned about the possible existence of another planet deep in the solar system.

Another boy wrote NASA about his interest in applying for a job as a planetary protection officer, defending Earth from biological contaminants. Sometimes, Green said, he receives drawings and paintings.

"It's really a delight for me," Green said. "It's a break in my day when I open up a letter like that and have an opportunity to see where these kids are at and spend time to actually answer their question."

Cara, in particular, impressed Green with her knowledge about the Kuiper belt.

"She got that. She understood that," Green said. "It was really exciting, these kids that really want to dig in and understand our solar system. I get really excited about it."

Carly Howett, a Colorado-based scientist involved in NASA's New Horizons mission to explore Pluto, also wrote a letter to Cara after finding out about the girl from an astronomy magazine in Ireland.

"I felt that that could've been me when I was her age. Reaching out and not really expecting much," said Howett, who grew up in a small town in the United Kingdom.

"It's important that that dream of hers, that spark not be extinguished."

"I kind of hope that this story shows that scientists don't have to be unreachable, that we're people," she added. "If you have a question, you reach out to whoever it is, whether it's Neil deGrasse Tyson or someone else. That's how curiosity starts."

The International Astronomical Union, not NASA, downgraded Pluto's status to that of a dwarf planet because it failed to meet all criteria needed to be classified as a full-sized planet.

Pluto met two: It orbits around the sun, and it has sufficient mass to assume a nearly round shape. But it failed to meet the third, which says it needs to have "cleared the neighborhood" around its orbit.

This means there should be no other bodies that are comparable in size in a planet's vicinity in space.

Cara is far from alone in her desire to restore Pluto's status as a planet. Last year, a group of NASA scientists proposed expanding the definition of a planet.

The scientists suggested that "round objects in space that are smaller than stars," should be considered planets, USA Today reported.

Coming to America and becoming a NASA astronaut may sound like distant dream for a girl from southern Ireland, but Green, of NASA's Planetary Science Division, said Cara needs only to look at someone who had already done so.

Nicola Fox, who was born and raised in a small town outside of London, is now a scientist for NASA and Johns Hopkins University. Like Cara, Green said, Fox started as a little girl whose bedtime stories were about the moon landing.

"She's been here in the country for about 20 years. That started just like Cara does in terms of dreaming and thinking about this field," Green said. "I'm hoping that my letter was just a little tug in that direction."

The full letter is below:

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This article was originally published by The Washington Post.