给定一个英语句子，创建一个程序，可以计算句子中单词的数量。


输入句子：There are nine planets in the solar system.
程序如下：
```
sentence = "There are nine planets in the solar system."
word_count = len(sentence.split())
print("Number of words in the sentence:", word_count)
```
输出：
```
Number of words in the sentence: 9
```