Early in the 19th century, scientists began to speculate that the Earth, surrounded by
the frigid vacuum of space, was habitable because its atmosphere contained special molecules like CO ₂ and water vapor, molecules that can absorb heat rays emanating from the Earth and thereby trap its heat.
The Leiden scientists replicated the
frigid vacuum of interstellar
space, then introduced the chemicals found in cometary ice and hit them with ultraviolet light like that emitted by stars.