How to create a list of custom objects in GraphQL

I am currently playing around with a bunch of new technology of Facebook.

I have a little problem with GraphQL schemas. I have this model of an object:

{
        id: '1',
        participants: ['A', 'B'],
        messages: [
            {
                content: 'Hi there',
                sender: 'A'
            },
            {
                content: 'Hey! How are you doing?',
                sender: 'B'
            },
            {
                content: 'Pretty good and you?',
                sender: 'A'
            },
        ];
    }

Now I want to create a GraphQL model for this. I did this:

var theadType = new GraphQLObjectType({
  name: 'Thread',
  description: 'A Thread',
  fields: () => ({
    id: {
      type: new GraphQLNonNull(GraphQLString),
      description: 'id of the thread'
    },
    participants: {
      type: new GraphQLList(GraphQLString),
      description: 'Participants of thread'
    },
    messages: {
      type: new GraphQLList(),
      description: 'Messages in thread'
    }

  })
});

I know there are more elegant ways to structure the data in the first place. But for the sake of experimenting, I wanted to try it like this.

Everything works fine, besides my messages array, since I do not specify the Array type. I have to specify what kind of data goes into that array. But since it is an custom object, I don't know what to pass into the GraphQLList().

Any idea how to resolve this besides creating an own type for messages?