Your professor said 'research paper, not essay', but didn't explain what that means in practice. Here's the actual difference, with real examples of both, so you stop submitting one when they asked for the other.
The core difference
An essay argues a position. A research paper investigates a question through systematic evidence. Essays are author-driven; research papers are source-driven. You'll always have a thesis in both, but the way you support it is fundamentally different.
Length and structure
Essays: 500-3,000 words; intro / body / conclusion. Research papers: 3,000-10,000+ words; intro / literature review / methodology / findings / discussion / conclusion. Different shape because different goal.
Source expectations
Essays: 3-8 sources, mostly secondary. Research papers: 15-50+ sources, mostly primary (peer-reviewed studies, original data). You can write a B essay with 5 sources; you can't write a B research paper that way.
What changes in your writing voice
Essays: argumentative, persuasive, sometimes first-person. Research papers: neutral, analytical, almost always third-person, hedged language ('the data suggest,' not 'I believe'). Match the voice to the genre or lose points.
When in doubt, ask
If you genuinely don't know which one your professor wants, ask. Most US professors will spell it out clearly when asked. (And if you need help on either format, TutorsGallery USA writes both, to the exact standards US instructors expect.)
Key takeaways
- Essay = argue a position
- Research paper = investigate a question
- Essays: 3-8 sources, secondary
- Research papers: 15+ sources, primary
- Voice: persuasive vs analytical