Subscribe is not a function error

I am trying to subscribe to an observable from a service, it builds without error but I get the error "this.service.getBanners(...).subscribe is not a function" when viewing in the browser.

Service:

import { Injectable } from '@angular/core';
import { Observable } from 'rxjs';

@Injectable()

export class BannerService {

    banners: any = ['1','2','3'];

    constructor(
    ) {}

    getBanners(): Observable<any[]> {
        return this.banners;
    }

    setBanners(banners: any[]): void {
        this.banners = banners;
    }

}

Component:

import { Component, ViewEncapsulation, OnInit } from '@angular/core';
import { Observable } from 'rxjs';
import { BannerService } from './../banner/banner.service';

@Component({
    selector: '.banner',
    templateUrl: './banner.component.html',
    styleUrls: ['./banner.component.sass'],
    encapsulation: ViewEncapsulation.None
})

export class BannerComponent implements OnInit {

    banners: any[];

    constructor(private bannerService: BannerService){

    }

    ngOnInit() {
        this.bannerService.getBanners().subscribe(banners => this.banners = banners);
    }
}

Any ideas what I am doing wrong?